Terraform Registry Support: Add support for token configuration that terraform supports.

[yorinasub17]

The initial version of Terraform Registry support introduced an environment variable for providing API tokens to terragrunt, but this is not how terraform authenticates. We should update the feature to support parsing the terraform authentication configuration.

Note the following caveat from the original PR:

Ideally we would be able to reuse code from Terraform. However, terraform has moved to packaging all its libraries under internal so that you can't use them as a library outside of Terraform. To respect the direction and spirit of the Terraform team, I opted for not doing anything funky to workaround the limitation (like copying those files in here).

[begemotik]

Would be nice to get it implemented.

[k2m30]

Gentlemen, any updates?

[pasinski]

Upvoting it too.