Move initialization of the ExtensionManager into the c'tor

[gvkries]

The initialization of the ExtensionManager is synchronous and must always be run before any method can be called. It can therefore be moved into the constructor to avoid a check before any method call. This is a minor performance optimization, but EnsureInitialized was called at least once during every request and on some requests dozens of times.

I based this optimization on PR #16379.