Add typed property getters mirroring topo_prop_get_* #16

Node::property returns a PropValue enum and asks the caller to pattern-match, which is the right interface when the property's type is unknown, but most callers know exactly what type they expect. As an example, in oxidecomputer/omicron#11279, omicron reads io/instance as a uint32 and ends up with a three-arm match covering the expected variant, any other variant with a warning, and the error case.

The C library has typed getters for this case, topo_prop_get_uint32, topo_prop_get_int32, topo_prop_get_int64, topo_prop_get_uint64, and topo_prop_get_string, and libtopo-sys already binds them. libtopo should surface them as Node::property_u32(group name), property_i32, property_i64, property_u64, and property_string, each returning Result<T, Error> and letting libtopo report a type mismatch as an error rather than having every caller reimplement the check.