Create simple mechanism to trigger codegeneration

[RickWinter]

Something to the likes of
cargo generate

[heaths]

For context, I don't think this should be in a build.rs file. Even if it's conditionally triggered on an env var or something, anyone using our crates will require extra build time to compile a build.rs file to an executable, run it, and then generate source that likely wasn't even regenerated. You can add files and env vars to the watch list, but this still require extra build time and Rust is slow enough to build already.

TypeSpecs shouldn't be changing often enough to warrant this, IMO. But I could see providing a cargo xtask task or similar, or even just a build script under eng/scripts that not only takes care of a platform-agnostic and non-PATH-specific mechanism to executable tsp-client (e.g., #1966 at the time of this writing is Windows- and PATH-specific), but could even take care of acquisition of tsp-client if easy enough to do in a platform-agnostic way.