feat: grpc-web plugin supports trailers as part of response body #15829

Job	Run time
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[a-k]*)	29m 43s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/plugin/[l-z]*)	30m 15s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/admin t/cli t/config-center-yaml...	20m 42s
build (ubuntu-20.04, linux_openresty, lua-resty-worker-events, t/node t/pubsub t/router t/script ...	21m 24s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[a-k]*)	28m 37s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/plugin/[l-z]*)	28m 34s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/admin t/cli t/config-center-yaml t/cont...	20m 31s
build (ubuntu-20.04, linux_openresty, lua-resty-events, t/node t/pubsub t/router t/script t/secre...	22m 59s
	3h 22m 45s

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