[New backend] Bitbucket Data Center OAuth2

[eshaan7]

The Bitbucket Data Center (formerly Bitbucket Server) OAuth2 flow is different from Bitbucket cloud (which is already supported). It would be a valuable addition to this package.

Reference: https://confluence.atlassian.com/bitbucketserver/bitbucket-oauth-2-0-provider-api-1108483661.html

[eshaan7]

I've implemented this in a private project already. Would generalize it and raise a PR!