Authn-JWT Refactor

[jvanderhoof]

This work is an effort to port the Authn-JWT functionality to the new Authenticator architecture.

Desired Outcome

Refactor Authn-JWT to use the new authenticator architecture defined in the new OIDC authenticator.

Implemented Changes

The authenticator has been completely ported, with all previous integration tests passing. I've added a Readme which overviews the architecture and interfaces.

Connected Issue/Story

N/A

Definition of Done

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: [insert issue ID]
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes