This pull-requests re-introduces the Bitbucket Server support with a
more or less complete rewrite of the forge implementation. We have a lot
of on-premises git repositories hosted in Bitbucket Server and need a CI
solution for running that and Woodpecker looks promising.
The implementation is based on external Bitbucket Server REST client
library which we are maintaining and have created in another context.
Besides the original support for Bitbucket the re-implementation also
adds support for handling Bitbucket pull-request events.
