Improve how we handle wallet_addEthereumChain with same chainID but different rpc urls

Context: https://github.com/MetaMask/metamask-extension/issues/15840

We should better communicate to users that the dapp is proposing to add a network they already have, but with a different rpc url and give users the choice to switch to the network they already have or adding it as a new custom network with the new rpc url - or maybe even replacing the current rpc url.
This ticket needs more definition and design before it's ready for development.

Start by reading the linked issue 15840 and reviewing the existing wallet_addEthereumChain flow. Define the user experience and design for choosing between switching to the existing network, adding a custom network with the new RPC URL, or replacing the current RPC URL before development begins.

Written by the indexing model from the issue text.