Generate only the needed subset of binaries for bindgen and proto

[dtolnay]

This removes, for example, the protobuf-bin-gen-rust-do-not-use binary that was previously getting exposed.

rules_rust/proto/3rdparty/crates/BUILD.bazel

Lines 77 to 81 in d6e3003

	alias(
	name = "protobuf-codegen__protobuf-bin-gen-rust-do-not-use",
	actual = "@rules_rust_proto__protobuf-codegen-2.8.2//:protobuf-bin-gen-rust-do-not-use__bin",
	tags = ["manual"],
	)

I had to change impl Add for CrateAnnotations to make crate-level gen_binaries work. It turns out in #1718 I didn't fully understand what this Add impl was supposed to do, and I had only tested gen_binaries before adding the global-level generate_binaries argument (#1718 (comment)). (I'm still not sure that I understand what this Add is supposed to do exactly, but at least now it appears to work in practice.)

The behavior I observed before the change in this PR was that global-level generate_binaries = True would do the right thing but crate-level gen_binaries would seem to get ignored whether or not global-level generate_binaries was set.

[UebelAndre]

You’re doing the lord’s work here. Thank you so much!