Add Serai RPC to the Serai node

[kayabaNerve]

Over the JSON-RPC, Serai nodes should offer the following calls:

1. Get current address for network. This should be in its fully encoded form, not the raw key we derive the encoded form from.
2. Get encoded InInstruction to perform a swap.
3. Simulate a swap path (already defined by the DEX pallet API).

Integrations with Serai as a DEX should be possible without any Substrate-specific calls/SCALE. That's why the existing DEX pallet API call is insufficient here (we need JSON formatted results, not SCALE).