allow/validate bitbucket_username / bitbucket_password in private config schema

[chris48s]

closes #8445

[shields-ci]

	Warnings
⚠️	This PR modified the server but none of its tests. That's okay so long as it's refactoring existing code.

	Messages
📖	✨ Thanks for your contribution to Shields, @chris48s!

Generated by 🚫 dangerJS against 774ba59

[ferarias]

looks good

[calebcartwright]

I have a very mild case of paranoia because I feel like there was something we did intentionally once upon a time relative to Bitbucket cloud, but I can't find any supporting evidence and don't recall any specifics so I do not have a high level of confidence.

This seems like a simple and reasonable enough thing to do, so I'll chalk my paranoia up to a weird dream 🤷

[chris48s]

I think its just oversight. The one thing I can think of where setting credentials for this could cause a problem would be if we weren't setting authorizedOrigins properly, but we're hard-coding an appropriate value in

shields/services/bitbucket/bitbucket-pull-request.service.js

Lines 66 to 73 in ae018cd

	this.bitbucketAuthHelper = new AuthHelper(
	{
	userKey: 'bitbucket_username',
	passKey: 'bitbucket_password',
	authorizedOrigins: ['https://bitbucket.org'],
	},
	config
	)

so I am not worried about this.

I think in general, the BitBucket badges are a bit of a patchwork: Not all of them work with both BB server and BB cloud. Not all of them support auth if you have credentials set. There's definitely some tidy up needed, but I don't think this causes any problem.