Add indexes to ci_builds_metadata

Ref: #373239 (closed)

What does this MR do and why?

Following up !97924 (merged) where we introduced 2 new indexes during low traffic this weekend.

This will be no-op operation gitlab.com.

This MR adds a migration which will create our indexes for users outside of gitlab.com as explained in our documentation.

Why are we doing this?

Those indexes are required in order to partition ci_builds_metadata as the primary key and all unique constraints should include the partition key in order to be able to partition our table.

Database queries

✅ Verification that our indexes exist on production

Table "public.ci_builds_metadata"
          Column           |          Type          | Collation | Nullable |                    Default                     | Storage  | Stats target | Description 
---------------------------+------------------------+-----------+----------+------------------------------------------------+----------+--------------+-------------
 project_id                | integer                |           | not null |                                                | plain    |              | 
 timeout                   | integer                |           |          |                                                | plain    |              | 
 timeout_source            | integer                |           | not null | 1                                              | plain    |              | 
 config_options            | jsonb                  |           |          |                                                | extended |              | 
 config_variables          | jsonb                  |           |          |                                                | extended |              | 
 interruptible             | boolean                |           |          |                                                | plain    |              | 
 has_exposed_artifacts     | boolean                |           |          |                                                | plain    |              | 
 environment_auto_stop_in  | character varying(255) |           |          |                                                | extended |              | 
 expanded_environment_name | character varying(255) |           |          |                                                | extended |              | 
 secrets                   | jsonb                  |           | not null | '{}'::jsonb                                    | extended |              | 
 build_id                  | bigint                 |           | not null |                                                | plain    |              | 
 id                        | bigint                 |           | not null | nextval('ci_builds_metadata_id_seq'::regclass) | plain    |              | 
 runtime_runner_features   | jsonb                  |           | not null | '{}'::jsonb                                    | extended |              | 
 id_tokens                 | jsonb                  |           | not null | '{}'::jsonb                                    | extended |              | 
 partition_id              | bigint                 |           | not null | 100                                            | plain    |              | 
Indexes:
    "ci_builds_metadata_pkey" PRIMARY KEY, btree (id)
    "index_ci_builds_metadata_on_build_id" UNIQUE, btree (build_id)
+    "index_ci_builds_metadata_on_build_id_partition_id_unique" UNIQUE, btree (build_id, partition_id)
+    "index_ci_builds_metadata_on_id_partition_id_unique" UNIQUE, btree (id, partition_id)
    "index_ci_builds_metadata_on_build_id_and_has_exposed_artifacts" btree (build_id) WHERE has_exposed_artifacts IS TRUE
    "index_ci_builds_metadata_on_build_id_and_id_and_interruptible" btree (build_id) INCLUDE (id) WHERE interruptible = true
    "index_ci_builds_metadata_on_project_id" btree (project_id)
Foreign-key constraints:
    "fk_e20479742e" FOREIGN KEY (build_id) REFERENCES ci_builds(id) ON DELETE CASCADE
Access method: heap

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