Github Importer - import pull request merged by

What does this MR do?

related to: #30731 (closed)

The merged by field is not available on the list of pull requests, which is used to import the merge request. Therefore, for each merged pull request we need to do another request to Github to fetch this field.

Once fetched, if the user can be mapped to a GitLab user, it's added as to the MergeRequest::Metrics#merged_by. Otherwise, a note is added to the merge request with the text: 'Merged by "login"', where "login" is the Github login of the user who merged the pull request in Github.

Screenshots (strongly suggested)

• When the user can be mapped to a GitLab user

• When the user cannot be mapped to a GitLab user

## 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