feat: url-pipeline core — parser, adapter ABC, registry, store hooks

[jhamman]

Summary

Implements URL pipeline support (https://github.com/jbms/url-pipeline): '|'-chained URLs resolve through pluggable adapters registered under the 'zarr.url_adapters' entry-point group (entry-point name = URL scheme).

• zarr.abc.url_pipeline: PipelineSegment, AdapterResolution, PipelineContext, URLPipelineAdapter (single-classmethod contract)
• zarr.storage._url_pipeline: parse_pipeline / resolve_pipeline; the root sub-URL delegates to make_store so existing file/memory/fsspec routing is unchanged
• registry: register_url_adapter / get_url_adapter / list_url_adapter_schemes (name check only; no adapter imports)
• make_store/make_store_path route strings containing '|' (or a registered root scheme) through the resolver; residual store paths combine with the user-supplied path
• StorePath gains a zarr_format attribute (populated by format segments in a follow-up)

For reviewers

This is the second of a series of Pull Requests toward zarr-developers#2943.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)