Add volume server RPCs to read and update state flags. (#8186)

* Boostrap persistent state for volume servers.

This PR implements logic load/save persistent state information for storages
associated with volume servers, and reporting state changes back to masters
via heartbeat messages.

More work ensues!

See https://github.com/seaweedfs/seaweedfs/issues/7977 for details.

* Add volume server RPCs to read and update state flags.

weed/server/volume_grpc_state.go

@@ -0,0 +1,26 @@
+package weed_server
+
+import (
+	"context"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb/volume_server_pb"
+)
+
+// GetState returns a volume server's state flags.
+func (vs *VolumeServer) GetState(ctx context.Context, req *volume_server_pb.GetStateRequest) (*volume_server_pb.GetStateResponse, error) {
+	resp := &volume_server_pb.GetStateResponse{
+		State: vs.store.State.Pb,
+	}
+
+	return resp, nil
+}
+
+// SetState updates state flags for volume servers.
+func (vs *VolumeServer) SetState(ctx context.Context, req *volume_server_pb.SetStateRequest) (*volume_server_pb.SetStateResponse, error) {
+	err := vs.store.State.Update(req.State)
+	resp := &volume_server_pb.SetStateResponse{
+		State: vs.store.State.Pb,
+	}
+
+	return resp, err
+}