Allow OAuth scopes in discovery document

[jceresini]

Following through after a feature request here: hashicorp/terraform#25354

This adds support for reading a list of scopes from the discovery document for an OAuth2 configuration. Scopes are optional for authz requests in OAuth2, but required for OIDC.

I added the tests requested in the linked feature request, however 2 of the existing tests fail for me with golang version 1.14.4. I saw an open PR to fix that already (#4), so I left it as-is in this PR

[hashicorp-cla]

All committers have signed the CLA.

[jceresini]

How does my employer go about signing a corporate CLA?

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[apparentlymart]

Thanks @jceresini! This looks great to me. Thanks for working on it!

While I was testing I also saw the failures relating to existing error messages include text from the Go standard library that has drifted in recent releases, so I added an extra commit here to address that so we can see for certain that all of the tests remain passing after your work.

I'm going to merge both of these commits now. I think the next step for your original request hashicorp/terraform#25354 would be to update Terraform itself to use a newer version of this library that includes these changes and then teach the terraform login support code how to handle Scopes as we were discussing over there.

The main Terraform codebase is currently in v0.13 release freeze, so we won't be able to merge this new feature in Terraform itself until the main branch reopens for v0.14 development in a few weeks, but I don't expect anything else related to terraform login to change in the meantime so if you're interested in working on it it should be safe to work against the current code on the main branch and in the unlikely event that some other conflicting changes do show up in the meantime we should be able to resolve the conflicts as part of reviewing it.

Thanks again!