Bitbucket Server: Modularize usage of Bitbucket client to support both Bitbucket Cloud and Server

[mickmister]

This ticket is related to both the API client task #121 and webhook processor task #122. Essentially the task here is to make common interfaces for the API client and webhook processor for Bitbucket Cloud and Server.

An example of a similar implementation can be found in the Jira plugin, which implements a common interface for Jira Cloud and Jira Server https://github.com/mattermost/mattermost-plugin-jira/blob/6109662e63375f22a332d0ce08bd0a6bcf3b7535/server/client.go#L33