Remove KV serialiser forward declarations

[jumaffre]

          I think it should be possible to remove this duplication, and thus remove the need for the `virtual void serialise(KvStoreMockSerialiser& s)` overload. I think that consumers of `kv_types.h` don't actually care about the templating of the args, so this could be a fully virtual type, and the templating (+duplicated naming) lowered to the implementation. But let's track that in a followup TODO, as it'll just add noise in this PR.

Originally posted by @eddyashton in https://github.com/microsoft/CCF/pull/5331#discussion_r1221596351