`ValidatedTopic` to prevent redundant topic validation

[CramBL]

Topics are validated on every publish e.g. in handle_publish

rumqtt/rumqttc/src/v5/client.rs

Line 90 in 005f73c

if !valid_topic(&topic) {

It would be great to avoid this overhead considering that topics are often known at compile-time.

Even though it's not a fantastically great overhead, I think the assembly shows that it is also not completely negligible: https://gcc.godbolt.org/z/evE4P88re

I'm aware of the history surrounding this change, namely:

• Empty strings are considered as valid topic #595
• RFC: rumqttc publish subscribe API design #774

Proposed solution

Reference implementation for v5: #980

Introduce a ValidatedTopic newtype that wraps a String, and use traits to differentiate it from regular String types. This is a solution that would maintain the existing API, allowing String/&str as the topic, but make it opt-in to use the "pre-validated" newtype for optimization.

Then we can also differentiate ValidatedTopic for v3(.1.1) and v5 by only allowing an empty string for the v5 version, which would resolve #595.