config: allow filesystem plugins to declare their own remote config schema via entry points

Problem

Third-party DVC filesystem plugins (installed via the dvc.fs entry point group) cannot be used as DVC remotes without modifying DVC core. The config validator in dvc/config_schema.py maintains a hardcoded list of URL schemes in REMOTE_SCHEMAS, so any plugin with a custom scheme gets rejected immediately:

dvc.config.ConfigError: config file error: Unsupported URL type osf:// for dictionary value @ data['remote']['myremote']

This was previously raised in #9711, which was closed due to lack of demand. I'm reopening as a concrete proposal with a working implementation.

Proposed Solution

Allow plugins to declare a REMOTE_CONFIG class attribute (a dict of config keys → validators) on their filesystem class. At import time, _discover_plugin_schemas() iterates over installed dvc.fs entry points and merges any declared schemas into REMOTE_SCHEMAS.

Plugin side (no DVC core changes needed by the plugin author beyond declaring the attribute):

class OSFFileSystem(ObjectFileSystem):
    protocol = "osf"
    REMOTE_CONFIG: ClassVar[dict] = {
        "token": str,
        "project_id": str,
        "provider": str,
        "endpoint_url": str,
    }

DVC core change (~25 lines in dvc/config_schema.py):

from importlib.metadata import entry_points

def _discover_plugin_schemas():
    for ep in entry_points(group="dvc.fs"):
        try:
            cls = ep.load()
        except Exception:  # noqa: BLE001,S112
            continue
        remote_config = getattr(cls, "REMOTE_CONFIG", None)
        if not remote_config:
            continue
        protocol = getattr(cls, "protocol", ep.name)
        schemes = (protocol,) if isinstance(protocol, str) else protocol
        for scheme in schemes:
            if scheme not in REMOTE_SCHEMAS:
                REMOTE_SCHEMAS[scheme] = {**remote_config, **REMOTE_COMMON}

_discover_plugin_schemas()

Properties

• ✅ Backward compatible — existing hardcoded schemes are never overwritten
• ✅ Zero overhead for users without third-party plugins (entry point group is empty)
• ✅ Graceful degradation — plugins that fail to load are silently skipped
• ✅ Minimal change — ~25 lines added to config_schema.py, no other files touched
• ✅ Tested — 7 new unit tests covering schema registration, skipping, multi-protocol, load failures, and ByUrl validation

Working Implementation

I have a working implementation in adamlabadorf/dvc@feature/plugin-schema-discovery, developed alongside dvc-osf, a plugin for Open Science Framework storage.

The dvc-osf integration tests (30 passing) confirm that with this change, dvc remote add, dvc push, and dvc pull all work correctly with a third-party plugin.

Happy to submit a PR — would appreciate any guidance on approach or concerns before doing so.

Start with dvc/config_schema.py, especially REMOTE_SCHEMAS and the proposed _discover_plugin_schemas() entry-point loading. Review the seven described unit-test cases for registration, skipping, multi-protocol support, load failures, and ByUrl validation. Done means third-party dvc.fs plugins can declare REMOTE_CONFIG without overriding existing schemes, and their remotes validate correctly.