feat: allow configuring allow-headers in grpc-web plugin #12630

Job	Run time
run ci on centos7 (lua-resty-worker-events, t/plugin/[a-k]*)	32m 14s
run ci on centos7 (lua-resty-worker-events, t/plugin/[l-z]*)	30m 15s
run ci on centos7 (lua-resty-worker-events, t/admin t/cli t/config-center-yaml t/control t/core t...	23m 15s
run ci on centos7 (lua-resty-worker-events, t/node t/pubsub t/router t/script t/secret t/stream-n...	21m 7s
run ci on centos7 (lua-resty-events, t/plugin/[a-k]*)	31m 51s
run ci on centos7 (lua-resty-events, t/plugin/[l-z]*)	31m 4s
run ci on centos7 (lua-resty-events, t/admin t/cli t/config-center-yaml t/control t/core t/debug ...	23m 0s
run ci on centos7 (lua-resty-events, t/node t/pubsub t/router t/script t/secret t/stream-node t/u...	21m 43s
	3h 34m 29s

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