Provide support for /api commands when working with Bitbucket Server

[pbaranow]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story
As a user, I'd like to use /api/plan endpoint for repositories hosted in Bitbucket Server.

Right now, when I issue curl command and use "Type": "BitbucketServer", Atlantis responds with:

{"error":"not yet implemented"}

I'd like to use this endpoint to implement drift detection for repositories hosted in Bitbucket Server

Describe the solution you'd like
It's up to the implementor, should you have any questions please let me know

Describe the drawbacks of your solution
N/A

Describe alternatives you've considered
I'm trying to detect drift, so my only option is to setup a complete environment with all variables and tools configured the same way as Atlantis.