Give the ScrubVolume() RPC an option to flag found broken volumes as read-only. (#8360)

* Give the `ScrubVolume()` RPC an option to flag found broken volumes as read-only.

Also exposes this option in the shell `volume.scrub` command.

* Remove redundant test in `TestVolumeMarkReadonlyWritableErrorPaths`.

417051bb slightly rearranges the logic for `VolumeMarkReadonly()` and `VolumeMarkWritable()`,
so calling them for invalid volume IDs will actually yield that error, instead of checking
maintnenance mode first.

weed/pb/volume_server.proto

@@ -656,6 +656,7 @@ message ScrubVolumeRequest {
     VolumeScrubMode mode = 1;
     // optional list of volume IDs to scrub. if empty, all volumes for the server are scrubbed.
     repeated uint32 volume_ids = 2;
+    bool mark_broken_volumes_readonly = 3;
 }
 message ScrubVolumeResponse {
     uint64 total_volumes = 1;