build: compile with C++20

[mpwarres]

std=c++20 is used by both V8 and Envoy. proxy-wasm-cpp-host needs to switch to use it in order to build with recent versions of V8. This will also reduce build surprises when importing proxy-wasm-cpp-host into Envoy.

This PR is stacked on top of #438, and obsoletes #411.

[leonm1]

I was able to successfully run env CC=/usr/bin/clang-12 CXX=/usr/bin/clang++-12 bazel build --config clang-tidy --define engine=multi --copt=-DPROXY_WASM_VERIFY_WITH_ED25519_PUBKEY=\"$(xxd -p -c 256 test/test_data/signature_key1.pub | cut -b9-)\" //... locally without any findings, so I will chalk the error up to a github runner issue and punt it to later / a separate issue.