Dispose httpclient ObjectCache for ApplicationLoadContext unloading

[nickeeex]

In .net framework AppDomains MemoryCaches are automatically registered to listen to AppDomain.OnUnload event and dispose there.
In dotnet this event is not fired so the MemoryCache is not disposed of automatically leading to a memory leak on the frends agent when the old Processes are not being evicted from memory.

For 5.5.4 this issue has been remedied partially directly in the product but this fix should still be implemented in the task itself.
Recommended ways to fix this is to create a static constructor for Frends.Web and listen to AppLoadContext(ALC) unload even and handle cleanup of resources there.

Consider that ALC's do not exist in .net framework and the task is multitarget.