`schemadiff`: formalize `InstantDDLCapability` (#14900) #304

Job	Run time
Build and push vitess/lite Docker images (latest)	3m 5s
Build and push vitess/lite Docker images (mysql57)	3m 0s
Build and push vitess/lite Docker images (mysql80)	3m 1s
Build and push vitess/lite Docker images (percona57)	2m 55s
Build and push vitess/lite Docker images (percona80)	3m 10s
	15m 11s

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