proposal: design Wasm ABI for tracing providers

Title: design Wasm ABI for tracers

Context:

• after Envoy moved to libc++, dynamic_ot-based tracers got broken because they were relying on C++ ABI
• following that, a potential new tracing provider expressed its interest in C ABI instead
• so, it seems that tracing providers could benefit from having their own Wasm ABI

Proposal:

• Let's try to design Wasm ABI for tracers
• Along the way, let's expand on ideas that came out of reviews for Wasm upstreaming PRs, namely:
  • ABI should be modular and complementary to proxy-wasm/spec
  • ABI should use versioning scheme similar to regular Envoy extensions, e.g. envoy.tracers.v1alpha0
  • ABI should support usage model consistent with regular Envoy extensions, e.g. it should be possible to have N instances of the same extension type each with a different configuration
  • ABI should be optimized for data passing from Envoy to the Wasm extension in just 1 function call