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.

• Cleaning up shared data proxy-wasm-rust-sdk#109
• Add the RemoveSharedData host call tetratelabs/proxy-wasm-go-sdk#307
• Add keys() and remove() methods to SharedData. proxy-wasm-cpp-host#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 proxy-wasm/proxy-wasm-rust-sdk#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.