mmr: move RPC from frame to client

[acatangiu]

Now that there is a dedicated client-side gadget with easy access to offchain db (#12753), move all offchain-specific functionality including proof generation from the pallet to the gadget.

This will greatly simplify the pallet which will no longer need both Storage<RuntimeStorage> and Storage<OffchainStorage> abstractions - it will simply have Storage (and will only work with runtime storage).

We should also move the MMR RPC code from frame/merkle-mountain-range/rpc to client/merkle-mountain-range/rpc; RPC is definitely not FRAME, but client-side.

[acatangiu]

After giving this a try, I'm not convinced it's actually better. We still need to call into runtime to get MMR size when generating proof (so it's not really more efficient), and we ultimately end up with proof generation and verification split across multiple components. I now think keeping proof generation in the pallet is better. The gadget is then responsible for only offchain canonicalization and pruning.

[acatangiu]

Fixed in #12805