Add backend support for Coverage-Check rule

What does this MR do?

Solves #15765 (closed)

This MR adds the backend support for the new MR approval rule named Coverage-Check.

Note that we only support coverage regex defined through the CI YAML config, and not through the project setting.

This MR is heavily inspired by !59698 (closed). The difference is that we do not want to rely on the coverage report artifact but rather compare the base and head pipelines coverage values.

Raw SQL

This is the SQL generated when fetching the builds in pipeline.update_builds_coverage:

SELECT
    "ci_builds".*
FROM
    "ci_builds"
WHERE
    "ci_builds"."type" = 'Ci::Build'
    AND "ci_builds"."commit_id" = 313784471
    AND "ci_builds"."coverage_regex" IS NOT NULL
    AND "ci_builds"."coverage" IS NULL

query plan: https://explain-depesz.postgres.ai/s/HeV

Screenshots (strongly suggested)

Sample screenshot when coverage is not lower than the base pipeline's:

Sample screenshot when coverage is lower than the base pipeline's:

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team