Support for cfg attributes in host functions definitions (#14189)

* Support cfg attribute in host functions definitions

* Added test to check feature gated methods are not included

* Versioned conditional compiled host function are forbidden

* Improve runtime-interface macro docs

* Fix doc

* Apply review suggestion

Co-authored-by: Koute <koute@users.noreply.github.com>

* Better error message

* Rust fmt

* More refinements to the docs

---------

Co-authored-by: Koute <koute@users.noreply.github.com>

substrate/primitives/runtime-interface/tests/ui/no_feature_gated_method.rs

@@ -0,0 +1,18 @@
+use sp_runtime_interface::runtime_interface;
+
+#[runtime_interface]
+trait Test {
+	fn foo() {}
+
+	#[cfg(feature = "bar-feature")]
+	fn bar() {}
+
+	#[cfg(not(feature = "bar-feature"))]
+	fn qux() {}
+}
+
+fn main() {
+	test::foo();
+	test::bar();
+	test::qux();
+}