Protoc command launcher

API to invoke protoc command from API (e. g. from build.rs), any

Note, protoc command must be in $PATH along with protoc-gen-LANG command.

Example of using protoc crate is in perftest's build.rs.

Note that to generate rust code from .proto, protoc-rust crate can be used, which does not require protoc-gen-rust present in $PATH.