Send request headers to the server and apply header mutations based
on the response. The rest of the protocol is still ignored.

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Make header mutation processing more robust and modular with unit tests.

Add stats to the ext_proc filter and add test assertions.

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Create a common function for the three places that we check
whether a header may be safely removed from a header map.

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Signed-off-by: Gregory Brail <gregbrail@google.com>

Ignore attempts to set the host, :authority, :method,
and any x-envoy headers. We may re-enable some of these later
when we can write more tests for them.

Signed-off-by: Gregory Brail <gregbrail@google.com>

Add ":scheme" to the list of headers that we will not set.
Use the proper mechanism to determine the right prefix for special
headers (usually x-envoy but it can be overridden)

Signed-off-by: Gregory Brail <gregbrail@google.com>