Support more custom RPC methods via Walletconnect

Following this issue - https://github.com/MetaMask/metamask-mobile/issues/2359,
MetaMask mobile currently supports only a few methods that called via Walletconnect: `eth_signTypedData`, `personal_sign`, `eth_sign`, `eth_sendTransaction`

Adding/Switching to custom network is missing and it's very convenient for users.

Start by reading the related issue #2359 and tracing the WalletConnect handling for the currently supported RPC methods: eth_signTypedData, personal_sign, eth_sign, and eth_sendTransaction. Determine where adding or switching custom networks is handled, then verify that the requested custom-network operations work through WalletConnect.

Written by the indexing model from the issue text.