add documentation BearerTokenResolver with snippets

[kostya05983]

Add docs to BearerTokenResolver with snippets, snippet is taken from existing code base.
gh-6254

[kostya05983]

@jzheaux , please see it.

[jzheaux]

Thanks, @kostya05983! I've provided some feedback inline.

[jzheaux]

When referring to class names, and other inlined code, please use backticks, e.g. BearerTokenResolver

[jzheaux]

Instead of showing the interface, could you show example usage? Specifically, I'm thinking it would be nice for the documentation to show how to do 1. form-based bearer tokens and 2. a custom header name.

[kostya05983]

@jzheaux , 1) Can you explain more about it? is It like Loosely Coupling? When we take parameters and generate a token? 2) We can provide a link to code with HeaderBearerTokenResolver.

[jzheaux]

Good questions.

For the first, there is a setting, DefaultBearerTokenResolver#setAllowFormEncodedBodyParameter. The spec allows for passing the bearer token via a form parameter instead of a header.

For the second (and in general), I recommend that the code be inlined, so the reader can see example usage in the context of your explanation. For example, you can imagine documentation written like this:

Or, we may need to use a custom header name, like when using Google Cloud's IAP Proxy authentication:

   http
       .oauth2ResourceServer()
            .bearerTokenResolver(new HeaderBearerTokenResolver("x-goog-iap-jwt-assertion"))

[jzheaux]

@kostya05983 thanks for your contributions here! I've merged your PR via 69a4848, and then I realized that it needed to be in the servlet docs, so I moved it and added a couple of examples via 5d0e80c