Received permission to make this crash public from @htuch and @gbrail

Title: Filter allows external processor to set 1XX status codes

Description:
The ext_proc filter does not validate changes to the response status code. This enables a processor to set the status code to 1XX and violate the FilterManager::encodeHeaders precondition.

Repro steps:
Adding the following test case to the integration test reproduces the crash.

// Test that the filter prevents an external process from changing the status code
// to 1XX. FilterManager::encodeHeaders() expects the status code is not 1XX and
// if this precondition is not held, envoy will crash.
TEST_P(ExtProcIntegrationTest, ResponseStatus100) {
  initializeConfig();
  HttpIntegrationTest::initialize();
  auto response = sendDownstreamRequest(absl::nullopt);

  processAndRespondImmediately(true, [](ImmediateResponse& immediate) {
    immediate.mutable_status()->set_code(envoy::type::v3::StatusCode::Continue);
  });

  // TODO downstream response status should not be 100
}

Call Stack:

[2021-08-03 00:58:24.424][1717][debug][filter] [source/extensions/filters/http/ext_proc/ext_proc.cc:393] Received immediate response response
[2021-08-03 00:58:24.424][1717][debug][http] [source/common/http/filter_manager.cc:909] [C739] [S17764435551718391193] Sending local reply with details
[2021-08-03 00:58:24.424][1717][critical][assert] [source/common/http/filter_manager.cc:1041] assert failure: !CodeUtility::is1xx(Utility::getResponseStatus(headers)) || Utility::getResponseStatus(headers) == enumToInt(Http::Code::SwitchingProtocols).
[2021-08-03 00:58:24.424][1717][critical][backtrace] [./source/server/backtrace.h:104] Caught Aborted, suspect faulting address 0xd40010000000c