Add dev zones for testing DNS seeders

[teor2345]

Motivation

There doesn't seem to be any way to test a newly launched ZFND seeder from Zebra.

We'd need to specify its IP address, and the canonical ZFND seeder DNS name, like:

dig @34.72.199.73 testnet.seeder.zfnd.org

But Zebra only uses tokio's basic DNS resolver API.

Design

We could add dev.testnet.seeder.zfnd.org and dev.mainnet.seeder.zfnd.org DNS names, so we can test development versions of the seeders before deployment.

(We'd need to change the seeder to serve the dev.* zones in addition to {test,main}net.seeder.zfnd.org. And then add those zones to our DNS records pointing at a fixed Google Cloud IP.)

Related Work

Discovered as part of:

• Re-deploy the foundation's DNS seeders for NU5 testnet #2797
• Add metrics for initial peer network protocol versions #2804

[mpguerra]

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