Support for on-demand data conversion in caches

Currently conversions between the storage format of a cache and the read/write format are done via a combination of TypeConverters, interceptors, converters and ad-hoc transformers.

In order to support reading and writing to/from a cache using multiple formats (e.g. read as Json, or as byte[]), it's necessary to isolate all those conversions and expose an API to specify on a per usecase basis the necessary conversion.