Implement Mock KV in-memory

[bogdanbiv]

When I've run denoflare serve on my worker I noticed Key-Value store is not implemented locally:

Error servicing request Error: KVNamespaceRpcStub.list not implemented.
    at RpcKVNamespace.list (blob:null/78de0f04-7a96-42c8-bc1a-2e61ce2dbfef:225:15)

I would need a mock KV for my tests so I'd like to try porting the mock implementation from edge-mock:
https://github.com/samuelcolvin/edge-mock/blob/11e6c03e3649d9c84cf1f1fad7bdabe9f86f9d18/src/kv_namespace.ts#L34

Does this sound like a good idea? Would this mock KV find an audience with the rest of Denoflare users?

My current idea is that the mock KV would be implemented as a part of a global this to be able to retain a reference to it between requests. Or maybe I could make it a part of some persistent Denoflare instance? For now I know I need to reuse typedefs KVNamespace, KVListResult, KVListResultKey from common/cloudflare_workers_types.d.ts

Please do comment.

[johnspurlock-skymethod]

This would be a great addition to denoflare proper, similar to the existing local memory R2 bucket implementation InMemoryR2Bucket which can be configured in local development with an R2 binding of local:memory:<bucket-name>

If you submit a PR with a new class InMemoryKvNamespace implements KVNamespace in pure TS with no dependencies, I'll wire it up to the local dev server so everyone can use it