[LG-267] Auth requests to decode pivcac tokens

[jgsmith-usds]

Why:
We don't want everyone being able to take a token and
inspect it. We only want the IdP able to do this.

How:
We have several options, including client/server TLS
certificates. But adding an HMAC with a shared secret
is the lightest weight way to lock down the decoding
endpoint.

N.B.: This is only half of the solution. We will need
to update the piv/cac server to expect the auth token
and verify it.

Hi! Before submitting your PR for review, and/or before merging it, please
go through the following checklist:

• For DB changes, check for missing indexes, check to see if the changes
  affect other apps (such as the dashboard), make sure the DB columns in the
  various environments are properly populated, coordinate with devops, plan
  migrations in separate steps.

• For route changes, make sure GET requests don't change state or result in
  destructive behavior. GET requests should only result in information being
  read, not written.

• For encryption changes, make sure it is compatible with data that was
  encrypted with the old code.

• For secrets changes, make sure to update the S3 secrets bucket with the
  new configs in all environments.

• Do not disable Rubocop or Reek offenses unless you are absolutely sure
  they are false positives. If you're not sure how to fix the offense, please
  ask a teammate.

• When reading data, write tests for nil values, empty strings,
  and invalid formats.

• When calling redirect_to in a controller, use _url, not _path.

• When adding user data to the session, use the user_session helper
  instead of the session helper so the data does not persist beyond the user's
  session.

• When adding a new controller that requires the user to be fully
  authenticated, make sure to add before_action :confirm_two_factor_authenticated.

[monfresh]

LGTM. Should this be marked do not merge until the server is updated?