Clarify function signature and return type

[jenniferjiangkells]

Currently the function signature for client and service functions is fixed:

• Service functions must take str as input and return Dict.
• Client functions must return an object of data generator type.

For service functions we can use the signature module and type check dynamically.
For client functions we're a bit more constrained atm - might have to make this more clear in the documentation.

Alternatively we may follow the principle that explicit = better and just have the users explicitly import the Pydantic models -> actually I think we should do this