Prepare foreign keys for bigint conversion on merge_request_diffs

What does this MR do and why?

Prepare foreign keys for bigint conversion on merge_request_diffs

• Ensure int to bigint backfill job is finished
• Synchronously create indexes to ensure their existence
• Create not valid foreign keys
• Queue asynchronous foreign key validation

This is a follow up for !195722 (merged)

Current table status

merge_request_diffs

Table "public.merge_request_diffs"
               Column               |           Type           | Collation | Nullable |                     Default                     
------------------------------------+--------------------------+-----------+----------+-------------------------------------------------
 id                                 | integer                  |           | not null | nextval('merge_request_diffs_id_seq'::regclass)
 state                              | character varying(510)   |           |          | NULL::character varying
 merge_request_id                   | integer                  |           | not null | 
 created_at                         | timestamp with time zone |           |          | 
 updated_at                         | timestamp with time zone |           |          | 
 base_commit_sha                    | character varying        |           |          | 
 real_size                          | character varying        |           |          | 
 head_commit_sha                    | character varying        |           |          | 
 start_commit_sha                   | character varying        |           |          | 
 commits_count                      | integer                  |           |          | 
 external_diff                      | character varying        |           |          | 
 external_diff_store                | integer                  |           |          | 1
 stored_externally                  | boolean                  |           |          | 
 files_count                        | smallint                 |           |          | 
 sorted                             | boolean                  |           | not null | false
 diff_type                          | smallint                 |           | not null | 1
 patch_id_sha                       | bytea                    |           |          | 
 project_id                         | bigint                   |           |          | 
 id_convert_to_bigint               | bigint                   |           | not null | 0
 merge_request_id_convert_to_bigint | bigint                   |           | not null | 0
Indexes:
    "merge_request_diffs_pkey" PRIMARY KEY, btree (id)
    "bigint_idx_248b52cc177d18a37d5e" btree (project_id, id_convert_to_bigint)
    "bigint_idx_9e3cffd9404ea9edfaac" btree (id_convert_to_bigint) WHERE files_count > 0 AND (NOT stored_externally OR stored_externally IS NULL)
    "bigint_idx_b82b85b8d4cd17894753" btree (merge_request_id_convert_to_bigint, id_convert_to_bigint)
    "bigint_idx_d108ffd7b24e5e2f8ab3" UNIQUE, btree (id_convert_to_bigint)
    "bigint_idx_d8fd566bc33f784a9129" UNIQUE, btree (merge_request_id_convert_to_bigint) WHERE diff_type = 2
    "index_merge_request_diffs_by_id_partial" btree (id) WHERE files_count > 0 AND (NOT stored_externally OR stored_externally IS NULL)
    "index_merge_request_diffs_on_external_diff" btree (external_diff)
    "index_merge_request_diffs_on_external_diff_store" btree (external_diff_store)
    "index_merge_request_diffs_on_merge_request_id_and_id" btree (merge_request_id, id)
    "index_merge_request_diffs_on_project_id_and_id" btree (project_id, id)
    "index_merge_request_diffs_on_unique_merge_request_id" UNIQUE, btree (merge_request_id) WHERE diff_type = 2
    "index_on_merge_request_diffs_head_commit_sha" btree (head_commit_sha)
Check constraints:
    "check_11c5f029ad" CHECK (project_id IS NOT NULL)
    "check_93ee616ac9" CHECK (external_diff_store IS NOT NULL)
Foreign-key constraints:
    "fk_56ac6fc9c0" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
    "fk_8483f3258f" FOREIGN KEY (merge_request_id) REFERENCES merge_requests(id) ON DELETE CASCADE
Referenced by:
    TABLE "merge_requests" CONSTRAINT "fk_06067f5644" FOREIGN KEY (latest_merge_request_diff_id) REFERENCES merge_request_diffs(id) ON DELETE SET NULL
    TABLE "merge_request_diff_commits" CONSTRAINT "fk_rails_316aaceda3" FOREIGN KEY (merge_request_diff_id) REFERENCES merge_request_diffs(id) ON DELETE CASCADE
    TABLE "merge_request_diff_files" CONSTRAINT "fk_rails_501aa0a391" FOREIGN KEY (merge_request_diff_id) REFERENCES merge_request_diffs(id) ON DELETE CASCADE
    TABLE "merge_request_diff_details" CONSTRAINT "fk_rails_86f4d24ecd" FOREIGN KEY (merge_request_diff_id) REFERENCES merge_request_diffs(id) ON DELETE CASCADE

References

#454893 (closed)

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Before	After

How to set up and validate locally

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Related to #454893 (closed)