heroku github auth

[wyattwalter]

Description

Provides authentication to API call back to GitHub from the Heroku review application build process. Auth isn't necessary to read the data we need (and the token has zero privileges), but helps us avoid rate-limiting.

Testing done

Only affects builds in Heroku, tested there

Unique to this PR

• authenticates requests back to GitHub

Applies to all PRs

• Appropriate logging
• Documentation has been updated, if applicable
• Provide link to originating GitHub issue, or connected to it via ZenHub
• Does not contain any sensitive information (i.e. PII/credentials/internal URLs/etc., in logging, hardcoded, or in specs)
• Provide which alerts would indicate a problem with this functionality (if applicable)

https://github.com/department-of-veterans-affairs/vets.gov-team/issues/12115

[ncksllvn]

LGTM!