Spark: Make FixedString type mapping configurable #777

Job	Run time
run-tests-with-specific-clickhouse (22.3)	4m 37s
run-tests (8, 2.12, 3.3)	4m 19s
run-tests-with-specific-clickhouse (22.8)	4m 20s
run-tests (8, 2.12, 3.4)	4m 25s
run-tests-with-specific-clickhouse (23.3)	4m 11s
run-tests (8, 2.12, 3.5)	4m 23s
run-tests-with-specific-clickhouse (23.8)	4m 16s
run-tests (8, 2.13, 3.3)	4m 22s
run-tests (8, 2.13, 3.4)	4m 8s
run-tests (8, 2.13, 3.5)	4m 4s
run-tests (17, 2.12, 3.3)	4m 24s
run-tests (17, 2.12, 3.4)	20m 32s
run-tests (17, 2.12, 3.5)	4m 23s
run-tests (17, 2.13, 3.3)	4m 15s
run-tests (17, 2.13, 3.4)	3m 58s
run-tests (17, 2.13, 3.5)	4m 15s
	1h 24m 52s

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