Bitbucket OAuth Support

[FizzBuzz791]

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I'm trying to publish my app with Bitbucket and from what I can see, the BitBucket Publisher only uses Basic auth. It'd be great if it could support OAuth and the client-cred flow. At present, I have to use my own account and generate an app password, which isn't the best flow.

[mmaietta]

Would love to have a community contribution on this. Setting up a local dev environment is fairly straight forward too https://github.com/electron-userland/electron-builder/blob/master/CONTRIBUTING.md#to-setup-a-local-dev-environment