Add option to use protobuf3

Users of the rust_protobuf module can use the 'use_protobuf3'
option to select version 3 of the protobuf crate instead of the
current default of version 2. This unblocks teams that would like
to use protobuf 3 immediately and provides a mechanism for us to
incrementally migrate users of protobuf 2 to protobuf 3.

Test: Adds tests that are executed during every build.
Bug: 270895633
Change-Id: Ib6a18b77cfa7dc3cc604cea05269004b3712bbb6

rust/testing.go

@@ -126,6 +126,12 @@ func GatherRequiredDepsForTest() string {
 			apex_available: ["//apex_available:platform", "//apex_available:anyapex"],
 			min_sdk_version: "29",
 		}
+		rust_library {
+			name: "libprotobuf",
+			crate_name: "protobuf",
+			srcs: ["foo.rs"],
+			host_supported: true,
+		}
 		rust_library {
 			name: "libprotobuf_deprecated",
 			crate_name: "protobuf",