Feature request: add values to DomainKeyUtil#getEnvironment()

[jak837]

Maybe in a future version there could be a way to add key/values to the map created in DomainKeyUtil#getEnvironment()?
This way developers can change DNS that is being used.
fx if a developer can add:
environment.put("java.naming.provider.url", "dns://8.8.8.8");
It use google dns for looking up the record.

Alternative a explicit dns api could be added to the library.

[bbottema]

I guess it makes sense to make the DNS host configurable. I would prefer to do that with explicit api, rather than a generic map.

[bbottema]

Implemented for 3.3.0 in commit 6f15c1f using the explicit API approach discussed above.

New API:

DomainKeyUtil.setDnsProviderUrl("dns://8.8.8.8");
String providerUrl = DomainKeyUtil.getDnsProviderUrl();

Passing null or a blank value resets lookups to the JVM default DNS provider. Changing the provider also clears the cached domain keys so stale lookups do not survive a resolver change. Added focused tests and a README usage note.