Prototype a contract that implements the reverse proxy pattern

[nagisa]

In order to scale contracts horizontally it is necessary to split it and the state up between shards. However that also implies that the contract will need to be called in a special way depending on what shard(s) the state is in, and require an extra effort from the contract developer to implement necessary state sharing between shards through cross-contract calls.

For the user-facing interface at least this can still be unified to an extent by having a contract that acts just like an nginx reverse proxy, using arbitrary logic to redirect the requests to a contract shard according to e.g. account id. This introduces an overhead of a cross-function call to each call but keeps all the nice properties that we expect of a single shard contract today (from the user API standpoint.)

We should draft an sample demo/example contract that contract developers could use to draw inspiration on how to scale past the confines of a single shard.