Implement v0.6 enum definitions

This does not implement accessors, just the enum definitions themselves.

PiperOrigin-RevId: 592591476

rust/BUILD

diff --git a/rust/BUILD b/rust/BUILD
index bc6a5cc..b3374f4 100644
--- a/rust/BUILD
+++ b/rust/BUILD
@@ -47,6 +47,7 @@
 #
 # shared.rs is the root of the crate and has public items re-exported in protobuf.rs for user use.
 PROTOBUF_SHARED = [
+    "enum.rs",
     "internal.rs",
     "macros.rs",
     "optional.rs",