Support for non-String keys in the rest server

The rest server assumes keys are always String, causing limited interoperability between remote endpoints.

When a cache is written via Hot Rod, by default keys will be stored as byte[] produced via JBossMarshaller (the default marshaller), or a UTF-8 byte[] in case the UT8Marshaller is chosen.

The Rest server should accept keys in different formats than String, using an encoding for byte[] values like Base64, and pass in an special header with the key media type so that the server can use it.

With this capability, compat mode could be avoided when writing from Hot Rod and reading from Rest and vice versa