Allow for multiple keysets configured

[johnlanda]

Overview

This change allows for the JWT auth backend to be configured with multiple JWKS URLs as the result of a customer request.

This change should allow for current configuration using jwks_url and jwks_ca_pem to work as normal. Users may optionally choose to instead specify sets of jwks_pairs. If jwks_pairs are specified then the underlying Validator from the cap library will loop over the available jwks_pairs in order to attempt to validate the JWT.

Design of Change

This change updates the config for the jwtAuthBackend in the path_config.go file. It also updates the backend.go file to ensure that when using jwks_pairs we iterate over the JWKS uris and CA PEMs to initialize all key sets.

Related Issues/Pull Requests

[ ] Cap PR #128

Contributor Checklist

[ ] Add relevant docs to upstream Vault repository, or sufficient reasoning why docs won’t be added yet
My Docs PR Link
Example
[ ] Add output for any tests not ran in CI to the PR description (eg, acceptance tests)
[x] Backwards compatible

[austingebauer]

Leaving a couple of small comments as I'm looking through this. Looking good!

[austingebauer]

Thanks again for this contribution, @johnlanda! LGTM