Add support for rustc flags to rust_proto_library

[UebelAndre]

When updating the version of Rust in #1930 I came across an error in the proto rules

(13:12:48) ERROR: /workdir/test/proto/BUILD.bazel:17:19: Clippy test/proto/b_rust.proto.rust/b_rust.clippy.ok failed: (Exit 1): process_wrapper failed: error executing command (from target //test/proto:b_rust)
  (cd /var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/sandbox/linux-sandbox/1824/execroot/rules_rust && \
  exec env - \
    CARGO_CFG_TARGET_ARCH=x86_64 \
    CARGO_CFG_TARGET_OS=linux \
    CARGO_CRATE_NAME=b_rust \
    CARGO_MANIFEST_DIR='${pwd}/test/proto' \
    CARGO_PKG_AUTHORS='' \
    CARGO_PKG_DESCRIPTION='' \
    CARGO_PKG_HOMEPAGE='' \
    CARGO_PKG_NAME=b_rust \
    CARGO_PKG_VERSION=0.0.0 \
    CARGO_PKG_VERSION_MAJOR=0 \
    CARGO_PKG_VERSION_MINOR=0 \
    CARGO_PKG_VERSION_PATCH=0 \
    CARGO_PKG_VERSION_PRE='' \
    CLIPPY_CONF_DIR='${pwd}/tools/clippy' \
    SYSROOT=bazel-out/k8-fastbuild/bin/external/rust_linux_x86_64__x86_64-unknown-linux-gnu__nightly_tools/rust_toolchain \
  bazel-out/k8-opt-exec-2B5CBBC6/bin/util/process_wrapper/process_wrapper --arg-file bazel-out/k8-fastbuild/bin/external/rules_rust_proto__libc-0.2.139/libc_build_script.linksearchpaths --arg-file bazel-out/k8-fastbuild/bin/external/rules_rust_proto__protobuf-2.8.2/protobuf_build_script.linksearchpaths --subst 'pwd=${pwd}' --touch-file bazel-out/k8-fastbuild/bin/test/proto/b_rust.proto.rust/b_rust.clippy.ok -- bazel-out/k8-fastbuild/bin/external/rust_linux_x86_64__x86_64-unknown-linux-gnu__nightly_tools/rust_toolchain/bin/clippy-driver bazel-out/k8-fastbuild/bin/test/proto/b_rust.proto.rust/lib.rs '--crate-name=b_rust' '--crate-type=rlib' '--error-format=human' '--codegen=metadata=-224398518' '--codegen=extra-filename=-224398518' '--out-dir=bazel-out/k8-fastbuild/bin/test/proto/b_rust.proto.rust' '--codegen=opt-level=0' '--codegen=debuginfo=0' '--remap-path-prefix=${pwd}=' '--emit=dep-info,metadata' '--color=always' '--target=x86_64-unknown-linux-gnu' -L bazel-out/k8-fastbuild/bin/external/rust_linux_x86_64__x86_64-unknown-linux-gnu__nightly_tools/rust_toolchain/lib/rustlib/x86_64-unknown-linux-gnu/lib '--edition=2018' '--extern=protobuf=bazel-out/k8-fastbuild/bin/external/rules_rust_proto__protobuf-2.8.2/libprotobuf-551824660.rlib' '-Ldependency=bazel-out/k8-fastbuild/bin/external/rules_rust_proto__byteorder-1.4.3' '-Ldependency=bazel-out/k8-fastbuild/bin/external/rules_rust_proto__libc-0.2.139' '-Ldependency=bazel-out/k8-fastbuild/bin/external/rules_rust_proto__iovec-0.1.4' '-Ldependency=bazel-out/k8-fastbuild/bin/external/rules_rust_proto__bytes-0.4.12' '-Ldependency=bazel-out/k8-fastbuild/bin/external/rules_rust_proto__protobuf-2.8.2' -Dwarnings)
# Configuration: f536cc10af391e0e7d5cac77a213b18582941d039d39ba1a143b8cea1b58f3ce
# Execution platform: @local_config_platform//:host
 
Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
error: ambiguous glob re-exports
 --> bazel-out/k8-fastbuild/bin/test/proto/b_rust.proto.rust/lib.rs:3:9
  |
3 | pub use a::*;
  |         ^^^^ the name `file_descriptor_proto` in the value namespace is first re-exported here
4 | pub mod b;
5 | pub use b::*;
  |         ---- but the name `file_descriptor_proto` in the value namespace is also re-exported here
  |
  = note: `-D ambiguous-glob-reexports` implied by `-D warnings`
 
error: aborting due to previous error

I've opted for a means to silence this warning vs fixing it because I'm generally unfamiliar with the proto rules and I suspect they will be replaced by @com_google_protobuf//rust in the future.

Thus, I've added rustc_flags which allow users to pass flags directly to the Rust compiler when building proto targets.