Add the host function for removing shared data

[mathetake]

This has been requested in several places, and the spec is the place where we need to solve it, so I raise this issue for visibility for future inquiries.

• https://github.com/proxy-wasm/proxy-wasm-rust-sdk/issues/109
• https://github.com/tetratelabs/proxy-wasm-go-sdk/issues/307
• https://github.com/proxy-wasm/proxy-wasm-cpp-host/pull/203
• envoy slack

[while1malloc0]

First off, thanks for the work on this, it's a super useful add to the spec.

Is there an externally facing place where this work is being tracked? I have a use case similar to https://github.com/proxy-wasm/proxy-wasm-rust-sdk/issues/109 in a memory constrained environment; we've worked around the issue by storing the entire map under a single shared k/v key and controlling for growth by manually manipulating that map, but obviously this is less than ideal so we're watching this issue with a lot of interest. The linked Slack conversations appear to be private, so I was just curious if there's anywhere that non-contributors are able to see any sort of timeline or progress on this issue.