Add dependency on mDNS rather than bundling it

[tomasz-wiszkowski]

Hello! Thank you for putting this library together!

I noticed that the library bundles in ESP8266mDNS dependency, rather than requesting it to be installed separately at specific version.

the outcome is that any project that would depend on both mDNS and notifier spots and reports a conflict that cannot be easily resolved.

It would be great if the library used __has_include feature (confirmed to work) (https://en.cppreference.com/w/cpp/feature_test) to report missing dependency, eg.

#ifdef __has_include
#  if !__has_include(<ESP8266mDNS.h>)
#    error Please use package manager to install ESP8266mDNS at version X
#  endif
#endif