Provide a mechanism for customising the auto-configured NimbusJwtDecoders

[wilkinsona]

Inspired by #20747, it looks like it would be useful to provide a mechanism for customising the auto-configured NimbusJwtDecoder. The use case in #20747 is to provide a RestTemplate to the decoder with custom proxy settings.

[mbhave]

I'd initially thought adding a JwtDecoderCustomizer would work but that's not the case. We would need a JwtDecoderBuilderCustomizer to support customizations. The problem is that the JwtDecoder built when using an issuer location isn't built using a builder so that could cause some confusion around what can be customized and what can't.

[wilkinsona]

I've just noticed that there also isn't a restOperations(RestOperations) method on the NimbusJwtDecoder.PublicKeyJwtDecoderBuilder. That makes sense as it doesn't make any HTTP calls. Perhaps #20470 isn't such a good driver for general purpose customization after all?

[mbhave]

Yeah, the public key version doesn't need a restOperations. The issuerUri one, however, does use one but it doesn't appear to be customizable. Given that, I don't think we should add a general purpose customizer for the JwtDecoder.

[m-kay]

Customization is only needed in the case if the JwkSetUriJwtDecoderBuilder is used, because all the other builders are fully configurable via properties. Therefore I think only a JwkSetUriJwtDecoderBuilderCustomizer would be needed as proposed in my branch gh-20750