Introduce ReactiveJwtAuthenticationConverter

[edeandrea]

Introduce an equivalent to JwtAuthenticationConverter on the reactive stack that users can override if they so choose.

Fixes gh-6273

[edeandrea]

I completed the proposed changes as well as rebased the branch into a single commit.

[jzheaux]

Thanks for all your hard work, @edeandrea! I think it's a great move to compose the Jwt to Collection conversion. I've left some feedback, let me know if you have questions.

[edeandrea]

I've pushed changes based on comments above & rebased the branch. You'll notice that JwtAuthenticationConverterTests now has fewer tests. Since the concept of converting a Jwt to GrantedAuthorities was abstracted out, I moved the majority of specific conversion tests into JwtGrantedAuthoritiesConverterTests. JwtAuthenticationConverterTests & ReactiveJwtAuthenticationConverterTests now just has a single default test along with a test for when overriding the JwtGrantedAuthoritiesConverter. One of the comments above mentioned doing that for ReactiveJwtAuthenticationConverterTests and I applied that to JwtAuthenticationConverterTests as well. If you'd like me to copy those tests back into JwtAuthenticationConverterTests I can surely do that.

[jzheaux]

Thank you, again, @edeandrea, this looks nice. I apologize, there are just two small items I missed in the last PR. I've left my feedback inline.

@rwinch, since this is a bit larger of a PR, please feel free to jump in with any other requests.

[edeandrea]

Changes made & rebase done.

[jzheaux]

Thanks for another PR, @edeandrea! This has been merged into master.