`schemadiff`: supporting textual diff (#15388) #544

Job	Run time
Build and push vitess/base Docker images (latest)	1m 9s
Build and push vitess/base Docker images (mysql57)	1m 7s
Build and push vitess/base Docker images (percona57)	1m 3s
Build and push vitess/base Docker images (percona80)	1m 3s
Build and push vitess/k8s image	1s
Build and push vitess components Docker images	1s
	4m 24s

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