schemachanger: support FastPathResults for zone configs

[annrpom]

In the legacy schema changer,

cockroach/pkg/sql/set_zone_config.go

Line 785 in 731e0f3

func (n *setZoneConfigNode) FastPathResults() (int, bool) { return n.run.numAffected, true }

is used to determine how many rows were affected for a CONFIGURE ZONE statement. In this case, we seem to just represent discard as 0 and any other write as 1. This issue tracks supporting something like this in the DSC

cockroach/pkg/sql/set_zone_config.go

Lines 1073 to 1077 in 731e0f3

	// We don't really care how many keys are affected since this function always
	// write one single zone config.
	if len(r.Keys) > 0 {
	numAffected = 1
	}

Jira issue: CRDB-44077

Epic CRDB-43532