[Rust] add function to retrieve file identifier

[rileylyman]

This adds a way for us to get the file identifier directly via Rust.

[ghost]

❤️‍🔥

[CasperN]

Sorry for the delayed review, I was on vacation.

Broadly LGTM but,

1. Can you add a test to tests/rust_usage_test/tests/integration_test.rs or somewhere?
2. (optional) Imo, we should be using [u8; 4] instead of &str+ assert. Flatbuffers requires 4 characters and we may as well make it a compile-time error. This does mean buffer_has_identifier needs to be marked deprecated and we'd need a new name.

[dbaileychess]

@rileylyman Any desire to continue with this?

[dbaileychess]

Closing for now, feel free to reopen.