Pick a single representation for incoming requests and responses

The testbench supports JSON, XML, and gRPC requests. Currently these are weaved throughout the code, with helper functions like this:

https://github.com/googleapis/storage-testbench/blob/57e6836e353b836eaae802df95d9e41f9bdd9c32/testbench/csek.py#L23-L30

To use the right fields for each request. I think we should normalize all the requests when received, where we know the exact format of the incoming request, and then use this single normalized form everywhere.