Frontend Improvement: Add network selector

[carlos-kryha]

Currently the rpc the frontend connects to is set via this const, this limits the frontend's funtionality to a single network. Having a dropdown to connect to arbitrary RPCs can improve UX by allowing people to use KREAd with any Agoric chain (including testnets and local sim-chains).

It is unclear atm whether we want to allocate design work for this, so for now the focus in on having a functional component that we can use internally, then we can later decide to make designs and update the style/position.

Acceptance criteria

• Network-config is store in Agoric Contex

• View dropdown component allows changing network-config to the following options:
  Emerynet -> https://emerynet.agoric.net/network-config
  Mainnet -> https://main.agoric.net/network-config
  Local testnet -> https://wallet.agoric.app/wallet/network-config

• const is still used for the default network (initial state)