Implements mocking of runtime apis (#5448)

* Implements mocking of runtime apis

This pr adds support for easily mock runtime api implementations for
tests by using the `mock_impl_runtime_apis!` macro. The syntax is
similar to `impl_runtime_apis!`. The mocked implementation automatically
implements `ApiExt`, `ApiErrorExt` and `Core` as these are required by
the runtime api traits, but not required in tests or only a subset of them.

* Fix warnings

* Update primitives/api/proc-macro/src/utils.rs

Co-Authored-By: Nikolay Volf <[email protected]>

* Review feedback

Co-authored-by: Nikolay Volf <[email protected]>