Prepare Ci::Build for partitioning

Ref: #382033 (closed)

What does this MR do and why?

This MR prepares Ci::Build for partitioning, similar to what we did to partition Ci::BuildMetadata.

Those indexes will be created during low traffic peak this weekend as ci_builds is one the busiest table 🤞

It creates 2 asynchronous indexes required to partition this table:

• Primary key: ci_builds: (id, partition_id).
• Unique constraint ci_builds: (token_encrypted, partition_id).

Indexes:
    "ci_builds_pkey" PRIMARY KEY, btree (id)
    "index_ci_builds_on_token_encrypted" UNIQUE, btree (token_encrypted) WHERE token_encrypted IS NOT NULL

In a follow up MR we will use swap_primary_key to make sure the primary key include the partition key.

This will create the following schema:

Indexes:
    "index_ci_builds_on_id_partition_id_unique" UNIQUE, btree (id, partition_id)
    "index_ci_builds_on_token_encrypted_partition_id_unique" UNIQUE, btree (token_encrypted, partition_id) WHERE token_encrypted IS NOT NULL

You can follow the documentation to create those indexes locally.

Why are we doing this?

Those indexes will be used to partition ci_builds as explained in this MR introducing zero partition helper.

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