Integrate support for FailOverRpcProvider into wallet-selector

[charleslavon]

Is your feature request related to a problem? Please describe.
Wallet-selector is currently hardcoded to use a single RPCProvider per network - https://rpc.mainnet.near.org and https://rpc.testnet.near.org

Describe the solution you'd like
Update wallet-selector's initialization method to leverage FailOverRpcProvider in setupWalletSelector and allow developers to specify multiple RPC Provider URLs such that wallet-selector instances can benefit from automatic failover to additional providers given service disruption in the first provider.

Acceptance criteria

• Unit tests should be added which verify the failover behavior
• Examples should be added to demonstrate how one should initialize wallet-selector in order to enable multiple RpcProvider

Additional context
FailOverRpcProvider was added into near-api-js here

[charleslavon]

hey @hcho112 @andy-haynes kindly review this request and provide any feedback