Add DDNS server example

[darkdragon-001]

Adjust the default values of the DDNS client example to connect to the one from the server example.

# Terminal 1
$ python3 examples/ddns_server.py
INFO:root:127.0.0.1: delete_rrset host.example. ANY A
INFO:root:127.0.0.1: add_to_rrset host.example. 300 IN A 10.0.0.5

# Terminal 2
$ python3 examples/ddns_client.py 10.0.0.5

[rthalley]

I don't understand the benefit of adding a server which doesn't do anything but print part of the DDNS message.

[darkdragon-001]

@rthalley This is an example. It took me quite some time to figure this out. Having an example would have been very helpful. Main motivation would be some sort of translator like https://github.com/pagekite/duppy, receiving RFC2136 requests and sending requests to proprietary DNS provider HTTP interfaces. They currently mix dnspython with async-python and monkey patching them together in https://github.com/pagekite/duppy/blob/main/duppy/dns_updates.py.

[rthalley]

I'm willing to merge something like this PR that simply provides an example asyncio-based mini-serve but does not change the library. It's a nice complement to nanonameserver for those who prefer asyncio.

I do not think that a general server framework is appropriate for the API at this time, but I'm not adverse to further discussion of it in discussion section.

[darkdragon-001]

I'm willing to merge something like this PR that simply provides an example asyncio-based mini-serve but does not change the library. It's a nice complement to nanonameserver for those who prefer asyncio.

Having such an example would have already saved me a lot of time. So if we can get this merged, it would be great.

I do not think that a general server framework is appropriate for the API at this time, but I'm not adverse to further discussion of it in discussion section.

Let's keep this PR with the example separate from the topic about a general server framework (discussion in #1247).