testing: fuzzing in Envoy Mobile

This issue discusses avenues for leveraging [fuzz testing](https://github.com/google/fuzzing/blob/master/docs/why-fuzz.md) in Envoy Mobile.

**Potential Areas for Fuzzing**:
1. Integration with the HCM: Envoy Mobile acts as a http codec interacting with the HCM -- `sendXXX` on the request path, and `onXXX` on the response path. [Structure Aware fuzzing](https://github.com/google/fuzzing/blob/master/docs/structure-aware-fuzzing.md) could help with detecting potential edge cases in API ordering, much like the Codec Fuzzer in upstream envoy.
Some things to consider:
- The fuzzer is not innately aware of illegal transitions, so initially there will be abundant false positives as illegal state transitions are found and pruned from the fuzzing path.
- Corpus for input fuzzing could be kept in sync with the stable API call patterns done in the `test/common/integration/dispatcher_integration_test.cc` test cases.
- BaseIntegrationTest (which is used by the dispatcher tests) is slow to fuzz, but google has a contractor working on performance.
- Mocks are challenging in fuzzing because they might be lacking functionality that leads to false positives. They are also slow to initialize (static initialization might help with this).
2. Integration with Filter Manager: Envoy mobile integrates L7 filters using the Platform Bridge filter, which translates state between the platform specific API calls and upstream Envoy's Filter Manager. This surface is also conducive to structure aware fuzzing. We might be able to draw inspiration from the upstream Envoy filter fuzzer.
3. Utility Classes: Envoy uses static utility classes to provide translation of data from one language runtime to another. Self differential fuzzing could be used to ensure correct roundtrip translation. The Envoy Mobile team has not experienced many crashes (and only some memory leaks) in this space.

**Project Ordering**
1. Setup basic fuzzing infrastructure using a simple use case. For instance, fuzzing inputs to utility classes.
2. Setup structure aware fuzzing for the API surface that integrates with the HCM drawing on prior art from Envoy's Codec fuzzers.
3. Setup structure aware fuzzing between Platform L7 filters (written in C++) that use the Platform Bridge Filter.

**Long term**
1. Explore complete end-to-end fuzzing from swift/kotlin.

**Additional Information**

The following examples were generously provided by @asraa. Any mistakes are mine.

False positives issues (beware these, or be prepared to handle these sorts of issues):
- Part of the issue is that the codec fuzzer fuzzes BOTH H/1 and H/2. Examples:
- H/1 requires connection close on stream reset so that was missing handling https://github.com/envoyproxy/envoy/pull/10643/files
- H/1 doesn’t allow multiple streams https://github.com/envoyproxy/envoy/pull/10452
- H/1 dispatches data after reset stream, another connection close/stream reset issue https://github.com/envoyproxy/envoy/pull/13014
- Parsers should already be rejecting some headers, caused issues in codec fuzzers https://github.com/envoyproxy/envoy/pull/10933