feat(ext/fetch): support custom DNS resolver

[zino23]

• Declare a new trait Resolve whose only method resolves a Name into Result<SocketAddrs, io::Error> asynchronously.
• Add a new variant to Resolver encapsulating a custom DNS resolver, and modify Resolver's implementation of Service<Name> accordingly.
• This new feature is tested in ext/fetch/dns.rs

Fix #27739.

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[zino23]

Hi @bartlomieju, could you take a look on this PR when you are available? I am sorry if I misuse the @ function on you.

[bartlomieju]

@zino23 what do you need this API for?

[zino23]

@zino23 what do you need this API for?

@bartlomieju We have a custom DNS rule that basically says for particular services, identified by a custom property in options of fetch api, instead of doing normal DNS resolutions, we directly connect to ips within a pool. These ips are assigned to local proxies located in the same data center as the JavaScript vms. The proxies will return resources cached in the data center, effectively speeding up the request.

The usage is like this:

// For requests that set `customProperty` to true, bypass the normal DNS resolutions.
await fetch("http://www.example.com", { customProperty: true })

[zino23]

@zino23 what do you need this API for?

@bartlomieju We have a custom DNS rule that basically says for particular services, identified by a custom property in options of fetch api, instead of doing normal DNS resolutions, we directly connect to ips within a pool. These ips are assigned to local proxies located in the same data center as the JavaScript vms. The proxies will return resources cached in the data center, effectively speeding up the request.

The usage is like this:

// For requests that set `customProperty` to true, bypass the normal DNS resolutions.
await fetch("http://www.example.com", { customProperty: true })

Besides, I guess others who need their custom DNS rules can also benefit from this?