Add Bitbucket OAuth provider

[aabajyan]

Hello, this is my first commit. If there's any issues let me know!

I tested with JS-SDK and I managed to sign in.
I ran make test and it seems to work fine.
I also ran make lint and no new errors was found.

You can create a new OAuth app here https://support.atlassian.com/bitbucket-cloud/docs/use-oauth-on-bitbucket-cloud/

[ganigeorgiev]

Thank you! I left some comments, feel free to check them when you find time.

I'll be able to test and review the integration in more details over the weekend.

[ganigeorgiev]

I was able to test the integration and everything seems to work fine!

I've squash-merged the PR locally in the develop branch with some minor modifications:

• added checks for the account_status:active field in case the account is suspended for some reason
• refactored the primary email function to ignore only the request errors for consistency with the other similar providers and to help with the debugging in case something else has gone wrong

The provider will be part of the next v0.21.0 release (there is no ETA yet).