feat(spanner): add getIsolationLevel and getReadLockMode methods to CommitResponse #20489

Job	Run time
windows	2m 21s
gapic-libraries-bom	21m 25s
bulk-filter	1m 2s
changes	1m 4s
lint	3m 14s
generation-config-check	1m 38s
enforcer	20m 30s
units (8)	1m 8s
units (11)	42s
units (21)	47s
units (17)	38s
units (25)	47s
split-clirr (java-spanner)	2m 22s
split-dependencies (java-spanner)	3m 28s
split-units (java-spanner, 25)	4m 12s
split-units (java-spanner, 8)	5m 13s
split-units (java-spanner, 21)	4m 28s
split-units (java-spanner, 11)	4m 43s
split-units (java-spanner, 17)	4m 21s
conditional-required-check	3s
	1h 24m 6s

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