Support injecting a custom DNS resolver to allow filtered IPs during MQTT connection

[machavarapun]

Problem Statement

Today, when connecting to an MQTT broker using a hostname, DNS resolution happens implicitly and the returned IPs are used directly for connection. While applications can perform pre-validation of DNS results (e.g., resolving the hostname first and filtering out restricted IP ranges), there is currently no way to pass a filtered/approved list of IP addresses into the MQTT connection itself.
This creates a gap where:

• A hostname may be validated ahead of time

• But a subsequent DNS resolution at connection time may still return a restricted IP

This is particularly relevant for scenarios where clients must connect to arbitrary or user-provided endpoints, such as connectors.

Proposed Enhancement

Allow callers to inject a custom DNS resolver into the MQTT client so that:

• DNS resolution occurs through caller-controlled logic

• Restricted IPs can be filtered out during resolution

• Only the approved IPs are returned and used for the connection attempt

Injecting the resolver would make DNS validation:

• Seamless (part of normal resolution flow)

• Safer (no mismatch between validation-time resolution and connect-time resolution)

• More efficient (avoids redundant DNS lookups)

[cartertinney]

Will consider this in future transport configuration work