Provide an option to opt-out of compile_well_known_types

[alexkirsz]

This PR adds a compile_well_known_types flag to the Prost toolchain declaration.

By default, for backwards compatibility reasons, this flag is set to True, as this is the current behavior. If this flag is switched to False, the compile_well_known_types option is no longer passed to Prost and Tonic, and the protoc wrapper will ignore types from the google.protobuf package when computing external types, relying instead on prost's own externals handling. Furthermore, the configured Prost types crate will be added as a runtime dependency to all rust_prost_library targets.

We're currently migrating away from our own set of proto rules to rust_prost_library, but our existing code relies heavily on the prost-types crate's implementations of well-known-types, which is the recommended way of using prost. However, the current behavior of rules_rust_prost makes it impossible to wire up properly, hence this PR.

Fixes #2058