Add Supplier JwtDecoders

[jzheaux]

Closes gh-9991

UPDATE - @rwinch and I chatted, and we'll leave gh-10309 out for now to keep the review smaller.

To use, you can publish a JwtDecoder like so:

@Bean 
JwtDecoder jwtDecoder() {
    Supplier<JwtDecoder> supplier = () -> JwtDecoder.withIssuerLocation("http://issuer/location");
    return new SupplierJwtDecoder(supplier);
}

Spring Boot could publish a SupplierJwtDecoder instead of a NimbusJwtDecoder.

[rwinch]

Thanks @jzheaux I provided feedback inline.