Add switcher for AutoScaler in disaggregated-tiflash mode

[guo-shaoge]

Enhancement

Originally, tidb fetch topo of tiflash_compute from PD when dispatching MPPTask. this pr #40729 add TopoFetcher to fetch topo from AutoScaler.

We need to add a switcer to fallback to old mode(fetch topo from PD) in case something bad happen in prod env.