metadata providers with matcher decorators

Matcher decorators are beatyfull, what think you about to support it for metadata providers?

It can look so:
```py
from typing import TypeVar

from libcst import BaseMetadataProvider, matchers as m
from libcst._nodes.module import _ModuleSelfT as _ModuleT

_ProvidedMetadataT = TypeVar("_ProvidedMetadataT", covariant=True)

class MatcherDecoratableMetadataProvider(m.MatcherDecoratableVisitor, BaseMetadataProvider[_ProvidedMetadataT]):
"""
The low-level base class for all non-batchable visitor-based metadata
providers. Inherits from :class:`~libcst.matchers.MatcherDecoratableVisitor`.

This class is generic. A subclass of ``MatcherDecoratableMetadataProvider[T]`` will
provider metadata of type ``T``.
"""

def _gen_impl(self, module: "_ModuleT") -> None:
module.visit(self)

```