Add configurable DNS client for ConnectionPool

[diegosalvi]

Fixes #6346

Motivation

Currently DNS client for PulsarClient cannot be configured and we are struck with default one. Specifically I needed to be able to configure the dns server list

Modifications

• Added a new method: ClientBuilder.dns
• Used configured dns builder instead of creating a new default one if provided in ConnectionPool

Verifying this change

• Make sure that the change passes the CI checks.

This change is essentially a trivial rework, it just expose the ability to provider a custom DnsNameResolverBuilder and use it if provided.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no) yes (added dependency on netty-dns-resolver to client api)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no) yes
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented) JavaDocs
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[sijie]

@diegosalvi the change looks good. although there is one small problem we need to think about - the pulsar-client is shipped by shading its dependencies. Hence DnsNameResolverBuilder will be shaded as well. so the end-users will be using the shaded class for configuring the DnsResolver.

So I would suggest defining a DnsResolverConfig in pulsar-client-api, and allow use configuring a DnsResolver by passing in a DnsResolverConfig. So we don't directly expose netty classes in Pulsar API.

[diegosalvi]

I'll add a configuration bean then :)

[codelipenghui]

@diegosalvi Any updates on this PR? Move this PR to 2.7.0 and If there is any update, move it back.

[benlongo]

Any updates on the state of this?

[tisonkun]

We support part of DNS configuration in #15219 by @lhotari.

To allow passing a DnsNameResolverBuilder in ClientBuilder, it can be a significant public interface change and I'd suggest go through a PIP process.

Closed as stale and conflict.