rumqttc: filters are not validated before subscribing

[swanandx]

Expected Behavior

Filters should be validated before subscribing and invalid filters should be rejected

Current Behavior

No validation is performed

Context

For publish, we check valid_topic before sending it to broker, we should do same for subscribe.

valid_filter is defined, but never used. ( same for v5! )

it should be used before sending the request, similar to how we do in publish.

note: might be required to validate at different place or at same place but with iter, as there might be multiple filters!