Add generic layer (e.g. trait) to enable supporting other binary formats (e.g. pgen)

### Description

https://github.com/chrchang/plink-ng/tree/master/pgen_spec

### Related Issues

_No response_

### Version

0

### Impact

Adding this feature could have the following drawbacks...

Adding this feature will introduce a breaking change...

Adding this feature will require updating the documentation...

### Suggestions

This feature could be implemented in a few different ways...

This feature should be tested in the following ways...

### Test input

_No response_

### Checklist

- [ ] Read the `CONTRIBUTING` guide
- [ ] Checked there are no existing Issues for this feature request
- [ ] Checked there are no Discussions that address this issue
- [ ] Checked you have installed at least the minimum required version of Rust
- [ ] Ensured that the latest release does not contain this feature
- [ ] Provided all information necessary to reproduce the current behavior

Start by reading the linked PLINK pgen_spec material and the repository's existing BGEN implementation to understand the binary-format assumptions. Define what a generic layer must expose for another format such as pgen, then add coverage demonstrating support for the new abstraction and update the documentation as needed.

Written by the indexing model from the issue text.