Adds metadata call functions retrieval

[thewhaleking]

Resolves #222

[basfroman]

Looks good!

wdyt about keeping the code consistent and creating classmethod in SubstrateMixin:

class SubstrateMixin:
...
    @classmethod
    def _get_metadata_call_funcion(cls, runtime: Runtime):

Then update the Async/SubstrateInterface.get_metadata_call_function call from self._get_metadata_call_function as you do now for get_metadata_call_functions?

[thewhaleking]

Looks good!

wdyt about keeping the code consistent and creating classmethod in SubstrateMixin:

class SubstrateMixin:
...
    @classmethod
    def _get_metadata_call_funcion(cls, runtime: Runtime):

Then update the Async/SubstrateInterface.get_metadata_call_function call from self._get_metadata_call_function as you do now for get_metadata_call_functions?

1. It shouldn't be a classmethod, as it doesn't utilise the class. See the other static methods in SubstrateMixin for reference.
2. While I would like to update the get_metadata_call_function, I want to keep API consistency with py-substrate-interface for methods that exist in it. This would break the return type, and thus also necessitate a major version update.