add_resolve_functions_to_schema should support Scalars parse_value and parse_literal

[rafalp]

Currently Ariadne Scalar support is limited to serializing python types to JSON before returning them to client, but we also want to support using custom scalars for input.

Our add_resolve_functions_to_scalar utility could support following use-cases:

Code below results in one-way only scalar:

• type_defs = {'Scalar': {'serialize': callable}}

And this code results in two-way scalar:

• type_defs = {'Scalar': {'serialize': callable, 'parse_value': callable, 'parse_literal': callable}} - explicit syntax for two-directional scalar.