support for merge request labels

[bzlibby]

• add remote methods
  • list requests
  • get request
• add command
  • args command
  • mode
• implement mode
  • for each request
  • resolve labels as usual

[ssube]

From https://docs.github.com/en/rest/reference/pulls:

Every pull request is an issue, but not every issue is a pull request. For this reason, "shared" actions for both features, like manipulating assignees, labels and milestones, are provided within the Issues API.

Given that, a sync-request command (Github and Gitlab differ on merge vs pull, but request is unambiguous) and some listRequest/getRequest methods on the Remote should be able to support this, using the same flags/states and logic as everywhere else.