Implement saving config content for pipelines

What does this MR do?

Besides .gitlab-ci.yml file, there are other factors that affect pipeline creation. So we need to store the content of its configuration.

This MR implements storing content of pipeline creation config in DB.

Storing this info was discussed as directly into ci_pipelines table, then it was decided to have this in a separate table.

ref: #38242 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers