Bitbucket Server: add token validator

[AlexTugarev]

Description

This PR implements a token validator for Bitbucket Server which is used to guess missing git permissions when using git.

Without this change every attempt to push a commit will cause this notification even if you are trying to push to your own repo:

Related Issue(s)

Fixes #9062

How to test

• push to a BBS repo with no write permissions => it should tell you that your token is missing write permissions.
• push to a BBS repo with write permissions => it should be quiet!
• repeat for GitLab or GitHub repos to verify this PR doesn't break the rest.

Release Notes

Bitbucket Server: implements token validation for Git operations.

[geropl]

@AlexTugarev I feel I'm doing sth wrong. I got this screent, depsite obviously being able to push, and having all permissions ticked (checked afterwards):

[AlexTugarev]

@geropl, thanks testing. is it https://bitbucket.gitpod-self-hosted.com/projects/FOO/repos/repo123/browse what you used for testing?

What do these resources tell you when navigation in browser while being logged in with a regular BBS user?
https://bitbucket.gitpod-self-hosted.com/rest/api/1.0/projects/foo/repos/repo123/permissions/users
https://bitbucket.gitpod-self-hosted.com/rest/api/1.0/projects/foo/permissions/users

[geropl]

@AlexTugarev I get this for bot requests (regular user):

[jldec]

Successfully tested all 3 cases as described in OP ✅

/hold in case you want to investigate what happened with @geropl

[geropl]

@jldec No, happy to see it worked for you 🙏 - not need to block this any further, then! 🎉

/unhold