feat: node discovery via DNS (#2045)

## Description

This enables global node discovery over DNS, i.e. dialing nodes by just
their node id.

Current setup is as follows:

* When dialing a node only by its NodeId, the new `DnsDiscovery` service
is invoked. It will lookup a TXT record at (by default)
`_iroh_node.b32encodednodeid.testdns.iroh.link` over regular DNS or
DNS-over-http. Right now the Cloudflare DNS servers are configured. At
`testdns.iroh.link` we run a custom [DNS
server](https://github.com/n0-computer/iroh-dns-server/tree/main)
* Nodes publish their Derp address to this DNS server through Pkarr
signed packets. This is an intermediate step, we decided that the
publishing by default should not happen by the nodes directly but
mediated through the Derp servers. Work for the latter happens in #2052

This PR thus allows for the following:
```sh
# terminal/computer 1
$ iroh console --start
Iroh is running
Node ID: qp2znfedwdij4llc5noizwfemfgba7bzxozvr4bp7hfsdmwqbpua
$ blob add ./myfile
...
Blob: o5uanh5s2zwn2sucy47puqidsfx2advxos7kajq3ajwitcwobhba
...

# terminal/computer 2
iroh console --start
blob get o5uanh5s2zwn2sucy47puqidsfx2advxos7kajq3ajwitcwobhba --node qp2znfedwdij4llc5noizwfemfgba7bzxozvr4bp7hfsdmwqbpua
```


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changes. -->

## Notes & open questions

* Misses node configuration in the CLI for the node origin domain (right
now hardcoded to `testdns.iroh.link`). How do we want to expose this -
CLI flag? Or in the config file? I'd say the latter.

* Offload publishing to the Derpers - see #2052 

* Right now the records published via pkarr have a TTL of 30s - the
iroh-dns-server will use that TTL as-is when serving the records over
DNS. both can/should change?

* We can also *very* easily allow to lookup nodes not only by NodeId,
but by any domain name. In the `iroh-dns` crate I included an example
`resolve` that does just that. By setting a `CNAME` record you can even
use any domain and simply point to the record hosted at the
`testdns.iroh.link` server.
So if, on your custom domain, you added a record like this
```
_iroh_node.frando.n0.computer CNAME _iroh_node.qp2znfedwdij4llc5noizwfemfgba7bzxozvr4bp7hfsdmwqbpua.iroh.link.
```
You can use this with the example to resolve to the node id and derp
addresses:
```
cargo run --example resolve -- domain frando.n0.computer
```

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## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.

Closes #1248

---------

Co-authored-by: Kasey <[email protected]>
Co-authored-by: Asmir Avdicevic <[email protected]>
Co-authored-by: Ruediger Klaehn <[email protected]>