Add put_reserved_with_flags to Database

[MukundaKatta]

Closes #366.

Database exposed put_with_flags and put_reserved separately but no combined version, forcing callers who need both reserved-key allocation and write flags to drop down to raw libmdbx. Mirrored the two existing methods into put_reserved_with_flags with the same signature shape and a doc-test.

Couldn't run cargo test locally in this sandbox; CI will verify.

[Kerollmops]

Thank you for your contribution. However, a little note about your comment: we don't rely on mdbx, and users cannot use raw LMDB calls to mirror what the method you implemented does.

Merging 🌵