Draft: add protos and generate code

[r-n-o]

This PR is a draft for codegen in the Rust SDK: internally Turnkey uses proto3 to define all types and interfaces so it seems like a waste to force our users to manually write types. We can generate it all!

The proto folder now contains the source protos, and src/gen contains generated code. I haven't yet tried to use this with a Rust project hence why this is still a draft PR.

[andrewkmin]

🏅 looking good!! just some minor comments to address

[andrewkmin]

would be nice to retain some semblance of a test suite with the rewrite :)

[andrewkmin]

do we need this in the case that there are other headers the user would like to include? or purely for testing?

[andrewkmin]

cc @joshhchun

[andrewkmin]

I'm not opposed to using a URL crate if it simplifies thing. any potential reason it'd be a bad idea?

[mountainpath9]

If this is intended to be a general purpose SDK, I'd suggest that accessing environment variables directly (and the use of .env files) should be up to the SDK user, and not baked into the SDK itself. My personal use case is likely to see these values coming from other rust code.

If you really want this for convenience, then env variable access should be optional, ie something like this this:

impl Turnkey {
    pub fn new(base_url: String, stamper: TurnkeyApiKey) -> Self {
        Self {
            base_url,
            stamper,
            client: Client::new(),
        })
    }

    pub fn new_from_env() -> TurnkeyResult<Self>  {
        Self::new(
           base_url: env::var("TURNKEY_BASE_URL")
                .map_err(|e| TurnkeyError::OtherError(e.to_string()))?,
            stamper: TurnkeyApiKey::new_from_env()?,
        )
    }