Add python code generation

[alexespencer]

My first rust contribution, this code:

• Allows --lang python to be passed to buffrs generate as well as an optional --out-dir argument which can redirect where the generated python code is exported to.
• Makes use of protocs --proto-path to give finer control over the location of generated code

[alexespencer]

Do we need to modify: https://helsing-ai.github.io/buffrs/integrations/cargo.html
Where we recommend build.rs is:

fn main() {
    buffrs::build(buffrs::Language::Rust).unwrap();
}

As Language::Rust has been removed from the enum, and lib.rs by default uses Tonic

[mara-schulke]

Yes!

[mara-schulke]

Left some things on the generation code, otherwise it looks good to me

[mara-schulke]

Last nits, let's merge once they are resolved :)