Enabled all *JwtDecoderBuilders to set a custom JWT validator

[ThomasKasene]

This was discussed briefly as an example to make a point in #13366, but as per a suggestion there, I went ahead and added a jwtValidator(...) method to all the *JwtDecoderBuilders to enable customizing the JwtDecoder and ReactiveJwtDecoder further without having to resort to awkward temporary variables:

@Bean
ReactiveJwtDecoder jwtDecoder(/* ... */) {
    return NimbusReactiveJwtDecoder
            .withIssuerLocation(...)
            .webClient(...)
            .jwtValidator(...)
            .build();
}

as opposed to this:

@Bean
ReactiveJwtDecoder jwtDecoder(/* ... */) {
    var jwtDecoder = NimbusReactiveJwtDecoder
            .withIssuerLocation(...)
            .webClient(...)
            .build();
    jwtDecoder.setJwtValidator(...);
    return jwtDecoder;
}

Disclaimer: I recognize that I was never given a "go ahead" in #13366 to do this, so I understand if this addition is not the desired direction for the project.

[ThomasKasene]

This is not quite in line with the team's wishes as it introduces a secondary way of accomplishing the same thing as the existing setter, which might be confusing to some according to this comment.

I'll keep the branch though, and maybe submit something for Spring Security 7.