Add sample ID getter for instrument region

[pomscyth]

Currently, some information about the sample used by an instrument region (e.g. sample ID, original pitch, sample rate, etc.) are not publicly available (being visible only within the crate). This PR addresses that by providing a way to access the sample directly via the sample ID.

[sinshu]

Thanks for the PR. This is indeed necessary 🤔 I think it would be better if the type returned by get_sample_id was usize, what do you think?

[pomscyth]

I'm okay, doesn't matter to me.