Add mechanism for pinning `flatc` to a specific version

As per discussion with @Tom-Willemsen and @DanNixon (a while ago now!)

We should pin the `flatc` version somehow to avoid generated bindings changing on new versions and/or when adding schemas.

This could be done by renaming `generate_rust_bindings.py` to `generate_bindings.py`, which goes and pulls a pinned binary of flatc from github, sticks it in the current working directory, then calls `flatc --python` and `flatc --rust` with the appropriate arguments

Start by reading generate_rust_bindings.py and the existing flatc invocation to understand how Python and Rust bindings are generated. Define the pinned flatc version and confirm how the binary is fetched from GitHub, then verify that both binding-generation commands still produce the expected outputs.

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