Allow setting a custom RPC in the UI

[mcintyre94]

If you want to use transaction detection on mainnet you need to use a custom RPC to avoid being rate limited. Currently you can only select the network and we use the default clusterApiUrl. We should add the ability to set a custom RPC URL in the UI so that developers can test transactions on mainnet easily using this scaffold.

As a workaround an endpoint can be set in the code: https://github.com/solana-labs/solana-pay-scaffold/blob/main/src/contexts/ContextProvider.tsx#L20