Prepare Ci::BuildMetadata for partitioning 1/3

Ref: #373239 (closed)

What does this MR do and why?

This MR prepares Ci::BuildMetadata for partitioning.

It creates 2 asynchronous indexes required to partition this table:

• Primary key: ci_builds_metadata: (id, partition_id).
• Unique constraint ci_builds_metadata: (build_id, partition_id).

Indexes:
    "ci_builds_metadata_pkey" PRIMARY KEY, btree (id)
    "index_ci_builds_metadata_on_build_id" UNIQUE, btree (build_id)

I added another index for build_id as I was getting the following error 👇

PG::FeatureNotSupported: ERROR:  unique constraint on partitioned table must include all partitioning columns
DETAIL:  UNIQUE constraint on table "p_ci_builds_metadata" lacks column "partition_id" which is part of the partition key

In a follow up MR we will use those indexed including partition_id to prepare this table to be partitioned as you can see in this example MR.

This will create the following schema:

Indexes:
    "ci_builds_metadata_pkey" PRIMARY KEY, btree (id, partition_id)
    "index_ci_builds_metadata_on_build_id_partition_id_unique" UNIQUE, btree (build_id, partition_id)

Why are we doing this?

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

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