mgr: narrow get_metadata return type with @overload

mgr: narrow get_metadata return type with @overload

Enable type narrowing for get_metadata() when a non-None default is
provided. Previously, the return type was always `Optional[Dict[str, str]]`,
forcing callers to use defensive `assert metadata` checks even when
a result was guaranteed.

The wrapper returns either the metadata from `_ceph_get_metadata()` or the
caller-supplied default. Providing an `@overload` allows type checkers to
prove the result is non-None, avoiding invalid assertions for falsy
defaults (like an empty defaultdict).

This is a hygienic change with no runtime impact.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>