Add facade types to avoid using raw Broker types

By avoiding to use `broker::data` directly, we gain a degree of freedom
that allows us to swap out `broker::data` for something else (e.g.,
`broker::variant`) in the future. Furthermore, it also helps us to keep
Broker types "local" to the Broker manager and gives us a nicer
interface.

Also replaces uses of `broker::expected` with `std::optional`. While an
`expected `can carry additional information as to why a value is not
present, nothing in Zeek ever cared about that. Hence, using
`std::optional` removes an unnecessary dependency on a Broker detail
while also being more efficient (no extra heap allocation when no value
is present).

src/script_opt/CPP/Func.h

@@ -83,7 +83,7 @@ public:
 
 protected:
     // Methods related to sending lambdas via Broker.
-    broker::expected<broker::data> SerializeCaptures() const override;
+    std::optional<BrokerData> SerializeCaptures() const override;
     void SetCaptures(Frame* f) override;
 
     FuncPtr DoClone() override;