Make blobs directly accessible through the graphql repository

What does this MR do?

Adds a new graphql field to the repository type to allow a set of blobs to be selected directly by path. The existing tree-based endpoints don't really allow for this.

An alternative approach would be to allow the tree's blobs field to have a paths argument , rather than opening a toplevel blobs next to tree. However, that would require the user to split the basename from the rest of the path to use the API. Exposing the blobs directly is easier.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a 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 to #323195 (closed)