[emscripten] Make filters and subscriptions easier to use from C++

[trev]

Background

We made filter and subscription classes easy to use from JavaScript, yet from C++ it is rather awkward. There is no easy way to convert a Filter instance to something like ActiveFilter safely, and only Filter references can be kept easily (no ActiveFilterPtr). Same issues apply to subscription, and one more: iterating over subscription's filters in C++ is quite complicated.

What to change

• Implement Filter::As<FilterSubClass>() that will either cast the filter to FilterSubClass or return nullptr if the instance is incompatible. Similarly, implement Subscription::As<SubscriptionSubClass().
• Define ActiveFilterPtr and the like in addition to FilterPtr.
• Expose begin() and end() methods on Subscription forwarding the corresponding methods from Subscription.mFilters.

[abpbot]

Some commits referencing this issue have landed:

• ​Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code
• ​Issue 5258 - Add typedefs for smart pointers referring to all filter and subscription classes