Implement generic mechanism to receive files via rest

As a prerequisite for a cleaner implementation of FLINK-9280 we should

• extend the RestClient to allow the upload of Files
• extend FileUploadHandler to accept mixed multi-part requests (json + files)
• generalize mechanism for accessing uploaded files in AbstractHandler

Uploaded files can be forwarded to subsequent handlers as an attribute, similar to the existing special case for the JarUploadHandler. The JSON body can be forwarded by replacing the incoming http requests with a simple DefaultFullHttpRequest.

Uploaded files will be retrievable through the HandlerRequest.

I'm not certain if/how we can document that a handler accepts files.