(macros::validate) support method args, and method- and property return values.

[luukvanderduim]

Currently the attribute macro is looking for signal types only.

zbus_lockstep provides functions and convenience declarative macros for signals, method args, method- and property return types.
zbus_lockstep_macros provides an attribute macro for signal types only.

It is a logical continuation to complete the set such that we may generate generates tests by decoration for each type.

• Signal types
• Method argument types
• Method return types
• Property return types