Standardize on u64 for token counts

[rtfeldman]

Previously we were using a mix of u32 and usize, e.g. max_tokens: usize, max_output_tokens: Option<u32> in the same struct.

Although tiktoken uses usize, token counts should be consistent across targets (e.g. the same model doesn't suddenly get a smaller context window if you're compiling for wasm32), and these token counts could end up getting serialized using a binary protocol, so usize is not the right choice for token counts.

I chose to standardize on u64 over u32 because we don't store many of them (so the extra size should be insignificant) and future models may exceed u32::MAX tokens.

Release Notes:

• N/A