add API to read job meta information via git ref

What does this MR do?

Issue: Backend: feature addition: add API to read job ... (#348789)

This MR allows to query the server for the most recent CI data of a certain job, linked to a given git reference.

This comes in particularly handy, if a project's pipeline executes some job conditionally. So, instead of fetching and filtering the project's pipelines to find one that contains the meta data of the desired job, one can simply query the server.

How to setup and validate locally (strongly suggested)

See the example given in the added documentation.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• 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 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