Rename Plugin to AuthenticationPlugin #767

Job	Run time
cs-stan / Coding Standard & Static Analysis	25s
testsuite / testsuite-linux (8.5, highest, false)	1m 19s
testsuite / testsuite-linux (8.3, highest, false)	37s
testsuite / testsuite-linux (8.2, lowest, false)	35s
testsuite / testsuite-linux (8.4, highest, false)	1m 20s
testsuite / testsuite-linux (8.2, highest, false)	34s
	4m 50s

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