This commit adds a new key that configures a GzipResourceResolver
in the resource handling chain.

Configuring an application with the following will add that resolver,
which checks for gzipped resources in the configured locations:

```
spring.resources.chain.gzipped=true
```

This means that if a resource "style.css" is requested, the
GzipResourceResolver will look for resources named "style.css.gz", which
should be a gzipped variant of the "style.css" file. Note that this
resolver only checks for variants if the client supports the "gzip"
encoding, as defined in the "Accept-Encoding" HTTP request headers.

Fixes #4683