Added stubs for webhook

[robbert229]

This is an example of what it may look like to add webhooks to this provider.

I need to write some tests for this pr one of these evenings, but until then @gavinbunney, does this new resource look sane?

[hartzell]

@robbert229 -- This looks useful.

• Are you still pursuing it?
• Can you add some supporting documentation?
• Does it rely on the Bitbucket webhooks plugin?
• Is there a plan for a matching data source?