[proxy] Fix proxy to be able to re-send request body

[addisonj]

Fixes #5360

This adds a small cache of the request body to ensure that it can be
re-sent.

[addisonj]

Okay, confirmed this works... not sure of an easy way to unit test it. I also want to add more one more change here to fix the strack trace on redirects that is confusing

[addisonj]

rerun integration tests
rerun cpp tests
rerun java8 tests

[merlimat]

not sure of an easy way to unit test it

I think it should be happening each time the proxy is handling a 307 and the request has a body.
In

pulsar/tests/integration/src/test/java/org/apache/pulsar/tests/integration/auth/token/PulsarTokenAuthenticationBaseSuite.java

Lines 250 to 254 in 7890750

	// Test multiple stats request to make sure the proxy will try against all brokers and receive 307
	// responses that it will handle internally.
	for (int i = 0; i < 10; i++) {
	admin.topics().getStats(topic);
	}

I was forcing multiple GET request to the proxy. The idea is that the proxy will round-robin the requests on the available brokers.

We could do the same for a PUT/POST request.

[merlimat]

run java8 tests

[addisonj]

run java8 tests

[addisonj]

rerun integration tests
rerun cpp tests

[sijie]

run java8 tests
run integration tests