Add mdnsresponder package and update openthread-br to use it by default

[ksperling-apple]

• Add mdnsresponder package

• Use mdnsresponder for openthread-br by default

  The new openthread-br-avahi variant of the package continues to use Avahi.

  Also tweak a few build options and default the UART baud rate to 460800; USB
  RCPs generally support this baud rate and it avoids the UART interface being
  the bottleneck for the border router.

[semanticdiff-com]

Review changes with  

[agners]

Nice, we use mDNSResponder for the Home Assistant OTBR add-on as well. I've upstreamed a mDNSResponder bump with some patches a while ago, I guess you have seen those patches (see https://github.com/openthread/ot-br-posix/tree/main/third_party/mDNSResponder)? Maybe some of these patches are also worth upstreaming? 🤔

[ksperling-apple]

Nice, we use mDNSResponder for the Home Assistant OTBR add-on as well. I've upstreamed a mDNSResponder bump with some patches a while ago, I guess you have seen those patches (see https://github.com/openthread/ot-br-posix/tree/main/third_party/mDNSResponder)? Maybe some of these patches are also worth upstreaming? 🤔

Ah, I didn't realize there was a version of this in the OTBR repo, I'll have a look at those.