feat: use self-signed JWTs in Spanner MutableCredentials #21615

Job	Run time
bulk-filter	1m 16s
changes	32s
units (8)	50s
windows	2m 18s
lint	3m 27s
enforcer	18m 59s
units (11)	33s
units (25)	38s
units (17)	36s
units (21)	32s
generation-config-check	1m 10s
gapic-libraries-bom	18m 54s
split-dependencies (java-spanner)	3m 55s
split-clirr (java-spanner)	2m 32s
split-units (java-spanner, 17)	4m 47s
split-units (java-spanner, 21)	6m 54s
split-units (java-spanner, 8)	4m 50s
split-units (java-spanner, 25)	4m 35s
split-units (java-spanner, 11)	5m 5s
conditional-required-check	3s
	1h 22m 26s

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