Adds option to configure authorization URL

[kennethormandy]

This adds the option to change the base URL, which is needed to support Stripe Connect with Express accounts. It leaves the current URL as the default.

It looks like at least one other person encountered changed this same thing on their fork, but from my understanding, this is the proper way for League providers to make that configurable.

The diff on the README license line is only for the line ending, sorry about that.

Thanks!

[kennethormandy]

It looks like the one failing CI build is due to this issue, rather than the PR specifically https://github.com/composer/composer/issues/8127