DNS configuration

[diegosalvi]

Is your feature request related to a problem? Please describe.
You can't configure any aspect of client DNS (servers, timeouts...)

Describe the solution you'd like
To ba able to configure at least DNS to use to resolve names

Describe alternatives you've considered
...

Additional context
I've dived into Pulsar client DNS internal configuration and I didn't found any way to properly configure DNS client properties on my system. Moreover if "autodiscovery" cannot find any dns it will attempt to use Google ones! In a corporate environment this isn't acceptable.

I could write you a patch to add dns configuration but I'm blocked between 3 alternatives:

1. add every needed dns configuration parameter on client configuration (... with some configuration bloating)
2. just require to provide an already configured netty dns and use current one as fallback (but it couldn't be configured via properties but only programmatically and expose internal netty dependences)
3. create a new "DNSClient" interface and a netty implementation and require to provide such dns object (but still cannot be configured via properties...)

[sijie]

2 seems to be a reasonable approach to me

[diegosalvi]

Created PR for (2)

[tisonkun]

Closed as stale. See #6382 (comment).