Add a context factory with GSN fallback for onboarding

[spalladino]

Add a factory and corresponding hook that creates a reasonable provider according to the following:

• If there is no injected provider (ie metamask), create a new key pair (or load one from local storage if found), and set up a GSN provider with it backed by a fallback provider (such as infura).
• If there is an injected provider, but without funds on the current network, create a new GSN provider that relies on the injected provider for signing (ie do not set a specific key, and let the RelayClient use eth_sign)
• If there is an injected provider with funds, use it.