Add support for a bitbucket datacenter integration

[leonhelmus]

At the moment only bitbucket cloud is possible to configure in Packeton.

I was wondering if there are already plans to integrate Bitbucket Datacenter as well. If not please let me know than i will try to contribute to add this functionality if appreciated.

This is the rest API that could be used:
https://confluence.atlassian.com/bitbucketserver0720/bitbucket-oauth-2-0-provider-api-1116282017.html

I already saw your pr:
#138

Which added support for bitbucket cloud should i create the same logic for bitbucket datacenter?

Please let me know how you like to approach this?