Add new GLib 0.14 variant types for metadata types

This way it's more convenient for downstream crates like ostree-rs-ext
to convert loaded variants.

TODO: Can we add a feature for the `gvariant` crate and expose via
that too?

rust-bindings/rust/tests/core/mod.rs

@@ -0,0 +1,13 @@
+use crate::util::*;
+use std::error::Error;
+
+#[test]
+fn variant_types() -> Result<(), Box<dyn Error>> {
+    let tr = TestRepo::new();
+    let commit_checksum = tr.test_commit("test");
+    let repo = &tr.repo;
+    let commit_v = repo.load_variant(ostree::ObjectType::Commit, commit_checksum.as_str())?;
+    let commit = commit_v.get::<ostree::CommitVariantType>().unwrap();
+    assert_eq!(commit.3, "Test Commit");
+    Ok(())
+}