[Feature Request] Support Bitbucket

**Is your feature request related to a problem? Please describe.**
We're trying to fully automate our process for creating and deploying azure resources and now reached the step to actually create build pipelines and deploy e.g. microservices. We rely on Bitbucket (Cloud) as our source code management system. This option is still not available with `az pipelines create` as it only supports github and tfsgit.

**Describe the solution you'd like**
Support bitbucket (cloud) as a provider when creating azure pipelines via CLI.

**Additional context**
This is also a blocker for e.g. https://github.com/microsoft/terraform-provider-azuredevops/issues/219