chore(spanner): reuse CharsetEncoder in ChecksumResultSet #3660

Job	Run time
filter	1m 9s
sdk-platform-java self-service clients	-1s
sdk-platform-java integration (JDK 24+)	-1s
gapic-generator-java (8)	-1s
unmanaged_dependency_check	-1s
sdk-platform-java units	-1s
clirr	-1s
dependencies	-1s
sdk-platform-java units (8)	-1s
sdk-platform-java integration	-1s
gapic-generator-java-bom	-1s
java8-compatibility	-1s
dependency_analyzer_unit_test	-1s
	57s

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