Adds "build_auth_token" option to Habitat Provisioner

[rwc]

Context: Per @bixu's comment (#16280 (comment)) on the original Habitat Provisioner PR, @nsdavidson and I moved forward with adding the ability to use private Habitat origins that require HAB_AUTH_TOKEN.

This PR:

• Adds the build_auth_token option in terraform and necessary functionality through which a user's HAB_AUTH_TOKEN can be passed.
• Updates the docs

[nsdavidson]

I have built this locally and verified that it works as expected. As mentioned, this was requested on original Habitat provisioner PR, and will be a must have for folks using private origins on the build service (https://bldr.habitat.sh).

Thanks @rwcee!

[jbardin]

Thanks @rwcee and @nsdavidson!

[bixu]

@jbardin, how can we track that this has been released in an official Terraform build?

[jbardin]

Hi @bixu,

With few exceptions, public releases are from the master branch, so the next tag and release will contain these changes. We don't have a set schedule, but it tends to be around a 2 week cycle, give or take depending on the changes made (which means we're about due for a release).

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