feat(authn): allow validation of jwt subject claim

[markphelps]

Re: #2985

• Adds support for validating subject in JWT claims
• Custom claim validation support will require more work as the library we use to validate JWTs does not support custom claim validation so we will need to do it ourselves

/cc @tstraley

[erka]

@markphelps. I would like to have a bit more clarity about it. Usually jwt sub is the unique user. Do we want to restrict access only for one person? Or should we allow the list of subjects in configuration?

[erka]

@markphelps It looks great. It would be nice to have two extra subtests in TestJWTAuthenticationInterceptor with testing sub

[markphelps]

@markphelps. I would like to have a bit more clarity about it. Usually jwt sub is the unique user. Do we want to restrict access only for one person? Or should we allow the list of subjects in configuration?

I would think so as well, however the JWT library we use that performs the validation also only validates a single subject: https://github.com/hashicorp/cap/blob/1b8eac34c97dffae4f63761b6848c8a4e5590d54/jwt/jwt.go#L164-L166

[GeorgeMac]

🕶️