Make `decl_runtime_apis!` fail on methods with default implementation (#7371)

* Make `decl_runtime_apis!` fail on methods with default implementation

Runtime api functions are not allowed to have default implementations.
This fixes this by throwing an error when we detect such a function.

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

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update primitives/api/test/tests/ui/no_default_implementation.stderr

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>