[VI-252] MAP STS token validation

[bramleyjl]

Summary

• This work is behind a feature toggle (flipper): YES/NO
• (Summarize the changes that have been made to the platform)
• (If bug, how to reproduce)
• (What is the solution, why is this the solution?)
• (Which team do you work for, does your team own the maintenance of this component?)
• (If introducing a flipper, what is the success criteria being targeted?)

Related issue(s)

• mockdata PR: https://github.com/department-of-veterans-affairs/vets-api-mockdata/pull/569

Testing done

• New code is covered by unit tests
• Describe what the old behavior was prior to the change
• Describe the steps required to verify your changes are working as expected. Exclusively stating 'Specs run' is NOT acceptable as appropriate testing
• If this work is behind a flipper:
  • Tests need to be written for both the flipper on and flipper off scenarios. Docs.
  • What is the testing plan for rolling out the feature?

Screenshots

Note: Optional

What areas of the site does it impact?

(Describe what parts of the site are impacted andifcode touched other areas)

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

Requested Feedback

(OPTIONAL)What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?

[bosawt]

We're going to want to cache this call, here is an example how we do it for id.me and login.gov public certs: lib/sign_in/public_jwks.rb.