fix: Load any stores.<name>.<attr> secret file (not just access_key/secret_key)

[dimitri-yatsenko]

Summary

Config._load_secrets() was hardcoded to only auto-load stores.<name>.access_key
and stores.<name>.secret_key from the .secrets/ directory. That whitelist made
sense when only the built-in s3 / gcs / azure stores existed, but it's wrong
for plugin-registered adapters (the storage-adapter plugin system added in
#1432) that define their own secret fields.

Concrete motivation

A plugin that wraps Databricks Unity Catalog Volumes authenticates via a Bearer
token. Its store spec wants a token field — not access_key / secret_key.
Today .secrets/stores.uc.token is silently ignored, forcing users to either:

• rename to a misleading access_key, or
• set the value programmatically at notebook startup (defeating the auto-load
  story DataJoint advertises in the configure-storage docs)

This change drops the whitelist. Any stores.<name>.<attr> file under the secrets
directory loads into dj.config["stores"][<name>][<attr>]. The existing precedence
is preserved — config file and env vars still win.

What changed

src/datajoint/settings.py — primary fix

- if attr in ("access_key", "secret_key"):
-     value = secret_file.read_text().strip()
-     ...
+ value = secret_file.read_text().strip()
+ ...

Comment around the loop updated to reflect the broader applicability.

src/datajoint/storage_adapter.py — incidental cleanup

Removed a try/except TypeError around entry_points(group=...). The fallback
was for Python < 3.10, where entry_points() returned a SelectableGroups dict
instead of accepting the group= kwarg. DataJoint's minimum supported Python is
3.10 (installation docs and
pyproject.toml's requires-python), so the branch is dead code — and was
producing a stale mypy error on SelectableGroups.get signature.

Tests

New test TestStoreSecrets.test_load_store_arbitrary_attr exercises both token
and api_key fields loading correctly. Existing tests
(test_load_store_credentials_from_secrets, test_secrets_do_not_override_existing)
still pass — the precedence semantics are unchanged.

Test plan

• Existing TestStoreSecrets tests still pass
• New test_load_store_arbitrary_attr passes
• All 23 tests in tests/unit/test_storage_adapter.py still pass
• Pre-commit hooks (ruff, mypy, codespell) green