Add a property to add an audience validator to resource servers JWT decoders

[ch4mpy]

Is your feature request related to a problem? Please describe.
Right now, with spring-addons starters, I have to override the complete authentication manager to just add an audience validator to resource server JWT decoders, which is rather cumbersome.

Describe the solution you'd like
I'd prefer to define an application property and have the starter autoconfigure an additional JWT validator for me. This would have two benefits:

• 0 Java conf for this validator
• I could adapt the audience to the environment (have different audiences in dev, staging and prod for instance