Allow for setting client ID / secret through files

[mpminardi]

Adds the ability to specify a path to the client ID and client secret in place of supplying their values directly in the provider.

[manicminer]

Thanks @mpminardi, happy to add this functionality. This change mostly looks good, I've tweaked & renamed a few things for consistency. I think we're going to have to run the tests using t.SetEnv() serially as env vars are process-wide, I'll add this similarly to how we do the same with other acceptance tests.

[mpminardi]

Thanks @mpminardi, happy to add this functionality. This change mostly looks good, I've tweaked & renamed a few things for consistency. I think we're going to have to run the tests using t.SetEnv() serially as env vars are process-wide, I'll add this similarly to how we do the same with other acceptance tests.

Sounds good, thank you for the cleanup and fixing the tests!

[jackofallops]

Thanks @mpminardi we and @manicminer - This LGTM 👍

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