[Proposal] Make ActiveMQ Classic more pluggable (Part 1)

docs/proposal/more_pluggable_activemq/broker_filter_change.md

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+## More pluggable ActiveMQ Part I : add more destination control on ActiveMQ Classic `BrokerFilter`
+
+### Problem statement
+
+ActiveMQ Classic natively supports plugin architecture through `BrokerFilter` and dependency injection at runtime defined in `activemq.xml`. There are already a few plugins shipped with ActiveMQ Classic main branch (such as `SimpleAuthenticationPlugin`) that users can configure on `activemq.xml` out of the box. Furthermore, it can support external plugins hosted outside of its main branch (see this [blog post](https://medium.com/p/9cafcb39fbe4) and the [public doc](https://activemq.apache.org/components/classic/documentation/developing-plugins) for more details). That said, there are ways to make the broker engine more pluggable. This design doc is the first part of a bigger discussion, to make ActiveMQ Classic more pluggable and grow its plugin ecosystem. This design doc proposes that the `BrokerFilter` should expose more destination level operations to be intercepted.
+### Rationale
+
+By exposing more destination-level events on `BrokerFilter`,a plugin will have more control and allows a plugin to support more features related to destination operations.
+
+### Proposal
+
+In the current state, here are the methods exposed in the `BrokerFilter` that intercept destination-level operations:
+
+1\. `addDestinationInfo`  
+2\. `removeDestinationInfo`  
+3\. `getDestinations`  
+4\. `getDestinationMap`  
+5\. `getDurableDestinations`  
+6\. `addDestination`  
+7\. `removeDestination`  
+8\. `virtualDestinationAdded`  
+9\. `virtualDestinationRemoved`
+
+We propose to add the following method to the `BrokerFilter`:
+
+1\. `purgeDestination()` method
+
+- This method is triggered when a destination is about to be purged.
+
+2\. `markDestinationForGC(long timeStamp)` method
+
+- This method is triggered when a destination is marked for GC
+
+3\. `browseDestination()` method
+
+- This method is triggered when a destination is browsed
+