Add client methods for DisconnectGitAlternates rpc

What does this MR do?

Adds client methods for DisconnectGitAlternates rpc. This will be used as part of object deduplication as an RPC that we will call when we notice an alternates file in a git repository that shouldn't have one. When called, the RPC will remove the .git/objects/info/alternates file.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry (not needed b/c this is a developer facing change)
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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