Enhancement: allow users to select network during onboarding, in Onboarding v2

[danfinlay]

MetaMask/metamask-extension#16696 importantly requests that we allow users to set a custom rpc during onboarding.

We should also allow users to select between our suggested networks during onboarding. This does not need to be resolved at the same time as the issue linked above, but can be.

We need to make it possible for the users to understand the implications of selecting different networks. Some of the providers are Infura, some are not.

Mobile version of MetaMask/metamask-extension#16708