Ensure build metadata record exists with before_validation

What does this MR do and why?

#387940 (closed)

This MR fixes the cascading of partition_id values to p_ci_build_metadata records for imports: #387301 (comment 1236757887)

More details in comments.

How to set up and validate locally

1. change the default values for all partitionable tables:
   • VERSION=20221220131020 bin/rails db:migrate:down:ci
   • comment out the Gitlab.com? check
   • VERSION=20221220131020 bin/rails db:migrate:up:ci
   • restart Rails and Sidekiq
2. export a project that has at least a pipeline with jobs
3. import it under a new name
4. check the database that all records(pipeline, builds, metadata) have the same partition_id value:

gitlabhq_development_ci=# select count(*) from ci_pipelines where project_id = 37 and partition_id = 101;
 count
-------
     0
(1 row)

gitlabhq_development_ci=# select count(*) from ci_pipelines where project_id = 37 and partition_id = 100;
 count
-------
    10
(1 row)

gitlabhq_development_ci=# select count(*) from ci_builds where project_id = 37 and partition_id = 100;
 count
-------
  2700
(1 row)

gitlabhq_development_ci=# select count(*) from ci_builds where project_id = 37 and partition_id = 101;
 count
-------
     0
(1 row)

gitlabhq_development_ci=# select count(*) from p_ci_builds_metadata where build_id in( select id from ci_builds where project_id = 37) and partition_id = 101;
 count
-------
     0
(1 row)

gitlabhq_development_ci=# select count(*) from p_ci_builds_metadata where build_id in( select id from ci_builds where project_id = 37) and partition_id = 100;
 count
-------
  2700
(1 row)

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