feat(splunk-logger): allow configuring keepalive_timeout (#11611) #774

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/http3/admin)	14m 6s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	35m 7s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	35m 24s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	25m 54s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	25m 34s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/http3/admin)	17m 51s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	35m 10s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	36m 49s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	28m 24s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	29m 24s
	4h 43m 43s

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