Verify C++ Wasm extension walkthrough guide about wasm-extensions HOT 2 OPEN

cc @mandarjog

from wasm-extensions.

Thanks for putting this together! I've been trying to walk through this myself. I'm running into a few issues:

1. The .bazelversion in the example repo does not always align with the version in the upstream istio repo where we fetch the latest envoy hashes as part of get-dep.sh. This leads to the script silently failing.
2. The WORKSPACE file is insufficent: it does not load some of the required modules. Perhaps it'd be better to just point to the example.wasm one?
3. I cannot get the integration test (example/test/example_test.go) to run. It cannot import the test module from this repo, which makes me suspect it intends to run as part of this repo. Since the walkthrough is geared towards somebody working in their own repo, it'd be helpful to either describe how to modify this file for your local repo, or move any test scaffolding out into its own go module.
4. I had to add a go.mod to the root of my repo to even start running the integration test. I've never really worked in a mixed cpp/golang env before, so I'm not sure if that's necessary or normal (is there a better way to initialise the dependencies?)

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