Add BitBucket build status support in bitbucket-server-plugin

Created by: ryanslade

The Bitbucket server plugin currently does not support sending build status related webhook events.

We'd like it to be able to send build status created and updated events: https://confluence.atlassian.com/bitbucket/event-payloads-740262817.html#EventPayloads-Buildstatuscreated

Related to: https://github.com/sourcegraph/sourcegraph/issues/7093