Added Stun and Cloudflare support

[MarkusTieger]

This PR adds 2 features:

• Adds a way to detect the outgoing public ip address (using a stun server.). Useful if it's behind a nat.
• Adds a way to update a cloudflare record without needing to write a http request every time

[rbi]

Hello @MarkusTieger , thanks for your contribution. It's nice to see that rddns seems to be useful for others.

I think the STUN server support is great to have in rddns. I would merge it with some minor fixes if it was a separate merge request.

The Cloudflare support would be the first provider specific extension. As it is in this MR it doesn't seem to be in the right module config. It also introduces several catch-all cases in match statements which makes it easier to miss them and implement bugs in case another DdnsEntry type is introduced later.

If it is implemented as DdnsEntry it should work like the other DdnsEntries HTTP and FILE. This means:

• Proper support DdnsEntry#resolvables and DdnsEntry#resolve
• Implementation of the update logic in a new method update_via_cloudflare in update_executers.rs. It can there use the plain hyper HTTP Client API directly like update_via_http does. There is no need for an intermediate step to first map to DdnsEntryHttp

Maybe in a future MR the update_executors can be broken up to sub-modules like it was done with the resolver module.

[MarkusTieger]

Maybe we should increment the version