ec: add diskType parameter to core EC functions

Add diskType parameter to:
- ecBalancer struct
- collectEcVolumeServersByDc()
- collectEcNodesForDC()
- collectEcNodes()
- EcBalance()

This allows EC operations to target specific disk types (hdd, ssd, etc.)
instead of being hardcoded to HardDriveType only.

For backward compatibility, all callers currently pass types.HardDriveType
as the default value. Subsequent commits will add -diskType flags to
the individual EC commands.

weed/shell/command_ec_encode.go

@@ -142,7 +142,7 @@ func (c *commandEcEncode) Do(args []string, commandEnv *CommandEnv, writer io.Wr
 		return fmt.Errorf("ec encode for volumes %v: %w", volumeIds, err)
 	}
 	// ...re-balance ec shards...
-	if err := EcBalance(commandEnv, balanceCollections, "", rp, *maxParallelization, *applyBalancing); err != nil {
+	if err := EcBalance(commandEnv, balanceCollections, "", rp, types.HardDriveType, *maxParallelization, *applyBalancing); err != nil {
 		return fmt.Errorf("re-balance ec shards for collection(s) %v: %w", balanceCollections, err)
 	}
 	// ...then delete original volumes using pre-collected locations.