[ok_http] Add support for client certificates using Java `PrivateKey`s #179

Job	Run time
health / breaking / run breaking health check	1m 37s
health / changelog / run changelog health check	1m 9s
health / coverage / run coverage health check	1m 11s
health / do-not-submit / run do-not-submit health check	1m 18s
health / leaking / run leaking health check	1m 24s
health / license / run license health check	1m 6s
health / comment	5s
	7m 50s

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