Introduce caching for functional routing

[JakobFels]

Currently the functional API does not support any kind of caching for resources. I was trying to replicate an nginx setup with multiple virtual hosts, each having their own assets folder.

So my code would contain something like:

route.nest(
  RequestPredicates.headers(headers -> "www.exmple.com".equals(headers.host().getHostString())),
  builder -> builder
    .add(ResourcesHandler.resources("/**", getHostSpecificResourceDir())

With this setup, I saw no option to define any kind of caching for these resources, because ResourceHandlerFunction was returning an EntityResponse which could not be modified.

This PR introduces caching support for the .resources instruction. Implementing the ResourceCacheLookupStrategy interface allows for either defining a general caching rule (e.g. max-age=60 for all resources), or to define custom cache behaviour per file (e.g. max-age=3600 if resource is an xml file).

The default behaviour, if no strategy is provided, is to not cache at all, preserving the status quo.

I have signed the CLA

[JakobFels]

Just wanted to know if anything is missing to get this moving

[poutsma]

Thank you for submitting a PR.

I decided to take an alternative approach (see 307a2c7) that allows you to change any headers for server resources, not just Cache-Control.