Add ability to create subdatasets per path/glob

Generated with the help of Claude Opus 4.7

## Summary

Extend `backups2datalad` to mint DataLad subdatasets for configurable path globs/regexes within a Dandiset, analogous to how `.zarr` directories are already handled as submodules.

## Motivation

This came out of a [discussion](https://github.com/dandi-compute/code/issues/87#issuecomment-4499305603) about how to organize derivatives and other large/modular components of a Dandiset without:

- Inflating inode usage in working trees (currently a full clone of e.g. `dandisets/000026` requires ~28,620 inodes even without zarr submodules, and annexed files require ~3x inodes each — easily hitting user inode quotas).
- Forcing users to clone entire `git-annex` tree just to work with a subset (sparse checkout can save local worktree).

The `.zarr` submodule pattern already demonstrates that fine-grained modular composition works well with DataLad. Generalizing it to arbitrary path patterns would let us apply the same benefits to derivatives, large NWB collections, or any other logically-separable subset.

## Proposed behavior

- Allow specifying one or more path globs/regexes (per-Dandiset, or via a default config) that should be split out as DataLad subdatasets.
- On backup, paths matching those patterns get their own subdataset (registered as a submodule in the parent), similar to the current `.zarr` handling.
- Subdatasets should be independently cloneable so that users can pull only the modules they need.

## Targets for Application

This would apply mainly to any BIDS derivative which has dandisets as an outer key

Note there is nothing preventing some non-DANDI maintainer from doing the same thing for some meta-analysis re-use as a BIDS-Study or derivative; surely we thus especially do not wish to encourage the 'duplicate the Dandisets' argument for a scaling of (number of Dandisets $\times$ number of reusers)

I suppose it could also WLOG apply to ANY/ALL outer subdirectories of monster Dandisets such as 000026 and could resolve some of the challenges faced over there