Return commits by author

What does this MR do?

Adds backend support for the ability to filter commits by the author field. This currently does not also support searching the message at the same time, as that is not supported in Gitaly. That can be added in a later iteration.

Notes

• This currently filters using the commit author name. I'll look at trying to do this via username in the next iteration, as it's more complicated
• This is currently only the backend implementation for this search. Frontend integration will come later

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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

Related #14984 (closed)