Add fieldMap support for dot notation in Identity::get() #820

Job	Run time
cs-stan / Coding Standard & Static Analysis	29s
testsuite / testsuite-linux (8.2, lowest, false)	30s
testsuite / testsuite-linux (8.5, highest, false)	1m 21s
testsuite / testsuite-linux (8.4, highest, false)	1m 17s
testsuite / testsuite-linux (8.2, highest, false)	33s
testsuite / testsuite-linux (8.3, highest, false)	33s
	4m 43s

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