Allow passing public RPC endpoint to the CLI

[0237h]

In graph init for example, if a start block lookup fails due to a faulty RPC endpoint, the user has to then input it manually.

It would be useful to have a CLI option that allows specifying a different public RPC endpoint from the hard-coded default.

Something like:

graph init --public-rpc <endpoint>

[YaroShkvorets]

With the upcoming registry release graph init will try all available RPC endpoints. Fault RPC endpoints shouldn't exist, and if that happens we should encourage making PRs to the registry instead.