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();
+}

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

@@ -0,0 +1,5 @@
+error[E0425]: cannot find function `bar` in module `test`
+  --> tests/ui/no_feature_gated_method.rs:16:8
+   |
+16 |     test::bar();
+   |           ^^^ not found in `test`

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

@@ -0,0 +1,12 @@
+use sp_runtime_interface::runtime_interface;
+
+#[runtime_interface]
+trait Test {
+	fn foo() {}
+
+	#[version(2)]
+	#[cfg(feature = "foo-feature")]
+	fn foo() {}
+}
+
+fn main() {}

substrate/primitives/runtime-interface/tests/ui/no_versioned_conditional_build.stderr

@@ -0,0 +1,5 @@
+error: Conditional compilation is not supported for versioned functions
+ --> tests/ui/no_versioned_conditional_build.rs:7:2
+  |
+7 |     #[version(2)]
+  |     ^