Support hint for 1 phase and 2 phase aggregate

[fixdb]

Enhancement

Currently we only have hint AGG_TO_COP to force pushing aggregate to TiKV.

It would be nice if we have hint of

• 1PHASE_AGG: Single phase aggregate, either on TiDB or TiFlash.
• 2PHASE_AGG: 2 phase aggregate. In case of TiKV, that will be TiDB final aggregate + TiKV partial aggregate. In case of TiFlash, that will be TiFlash final aggregate + TiFlash partial aggregate.

[fixdb]

/assign @Reminiscent