Add support for "approvals_syncing" MR status

[fbpe]

Fixes #31

Retrying on all "unknown" MR status is also not perfect:

• sometimes we have to consider the broken_status as valid. See Manage the "broken_status" in the MR status #21
• we know that some status will lead to a "mergeable" status, so we try merge already. For the moment, the list of "mergeable" MR status is: mergeable|ci_still_running|ci_must_pass|status_checks_must_pass, documented in
  https://unblu.github.io/ucascade/tech-docs/30_technical-documentation.html#_created_auto_mr

With all the sub-cases I went with just adding support for the new status approvals_syncing, but let me know your thoughts.