Split data model

[mversic]

logically, there is 3 apis:

1. client - server
2. server - executor
3. client - executor

4th is executor foreign functions and their payload. This one shouldn't be mixed with the previous ones. And this one doesn't have to be stabilized for the time being because it is low level and user is always required to use some of the provided macros to.

5th is iroha_schema where we could also say that schema doesn't need to be stabilized because users will always be required to use one of iroha clients but here I don't find that argument entirely convincing

Proposed changes

not sure. Let's figure out what is the scope of change needed here and if we need to do some refactoring

Motivation

structs and APIs that cannot be used by client shouldn't be accessible by the client, etc.