[Testing] Test Netty4HttpHeaderVerifier with different types of persistent connections

[cwperks]

While developing on #3418, it was noticed that the Security plugin does not have any tests around behavior with different types of http persistent connections.

#3418 Adds a new step in the netty pipeline called Netty4HttpHeaderVerifier that verifies a request and uses channel attributes to carry information to future steps in the pipeline. With persistent connections, only a single request/response can occupy a channel until the next request goes through the channel. In Pipelining and Multiplexing, http introduced some optimizations to increase the throughput of a channel. It would be useful to have tests in this repo to test different types of persistent connections.

MDN docs on connection management: https://developer.mozilla.org/en-US/docs/Web/HTTP/Connection_management_in_HTTP_1.x
HAProxy post on pipelining and multiplexing: https://www.haproxy.com/blog/http-keep-alive-pipelining-multiplexing-and-connection-pooling

• Tests Persistent Keep-Alive Connections (single request/response over same connection)
• Http1.1 Pipelining (Multiple requests can be fired, but must be responded to in order)
• Http2 Multiplexing (Ordering does not matter, same connection)

[scrawfor99]

[Triage] Hi @cwperks, thanks for filing this issue. Looks great, will add the triaged label :).