Simplify at+jwt validation

[jzheaux]

at+jwt validation was added in 6.x, and it would be nice if this were the simplest validation method to add, given that it is a standard for JWT validation.

Some ways to achieve this are:

• Add Nimbus(Reactive)JwtDecoder builders that adhere to RFC 9068
• Add Nimbus(Reactive)AtJwtDecoder implmentations that adhere to RFC 9068

This may require deprecating Nimbus(Reactive)JwtDecoder or some of their builders.

The DSL should also be considered. Perhaps something like:

oauth2ResourceServer((oauth2) -> oauth2
    .atJwt()
        .issuer(...)
        .audience(...)
)

Note that this ticket remains a work in progress and is not ideal for contribution at this time.