Add a `--network` flag to `fuel-indexer`

[ra0x3]

• We very often start the service connected to beta-4
• Using --fuel-node-host beta-4.fuel.network and --fuel-node-port 80 can be a bit much
• We should add a --network option that will automagically imply --fuel-node-host beta-4.fuel.network and --fuel-node-port 90
• Usage fuel-indexer --run-migrations --network beta-4

Additional context

• We should also then update all docs to reference --network beta-4

[deekerno]

This is a great first issue. 👌🏽

To a would be contributor: I actually implemented this functionality in the past, but that branch is loooooooong out of date. Feel free to use this updated snippet as a clue for your work:

lazy_static! {
    pub static ref AVAILABLE_NETWORKS: HashMap<&'static str, &'static str> =
        HashMap::from([
            ("Local Node", "127.0.0.1:4000"),
            ("Beta-4", "beta-4.fuel.network:80"),
        ]);
}