support fly wireguard create --network [custom-id]

[mattvr]

Change Summary

What and Why:

• Private networking support has been implemented for fly apps create and fly ssh console, but there's still no way with the CLI to locally connect to a custom network with wireguard.

How:

• Adds a --network flag which is passed through the wireguard create command.

Completes the following:

• Teach flyctl about app networks #417
• console / wireguard: add support for network  #3051
• Add custom network support to the agent. #3447

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Documentation

• Fresh Produce
• In superfly/docs, or asked for help from docs team
• n/a

[mattvr]

bump @billyb2 @benbjohnson