[Backend] Add a setting option for self-hosted instances to limit jobs in a pipeline

🧩 Problem Statement

In the issue #329926 (closed) a limit is being applied for .com on the allowed number of jobs in a pipeline for it to get created successfully. Self-hosted instances should have a freedom to set the limit by themselves.

💬 Background

#329926 (comment 584043536)

✨ Design proposal

Add a setting option for self hosted to enforce and set a limit for the number of jobs in a pipeline.

Backend Technical Proposal

Add a new column to project_ci_cd_settings. This should be an integer column, maybe the with the name builds_per_pipeline_limit.

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/project_ci_cd_setting.rb

Add the ability to get and update the field via the graphql API.

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/types/ci/ci_cd_setting_type.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/mutations/ci/ci_cd_settings_update.rb

Add the ability to get and update the field via the REST API.

• https://gitlab.com/gitlab-org/gitlab/-/blob/26720aaf4dc0c5e86dddb7fc96553db2c570c788/lib/api/projects.rb#L404

Update the application to check the default limit or the new field.

• Blocked on: #329926 (closed)