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/Func.h

@@ -18,17 +18,12 @@
 #include "zeek/ZeekArgs.h"
 #include "zeek/ZeekList.h"
 
-namespace broker {
-class data;
-using vector = std::vector<data>;
-template<class>
-class expected;
-} // namespace broker
-
 namespace zeek {
 
 class Val;
 class FuncType;
+class BrokerData;
+class BrokerListView;
 
 namespace detail {
 
@@ -231,14 +226,14 @@ public:
      *
      * @return a serialized version of the function's capture frame.
      */
-    virtual broker::expected<broker::data> SerializeCaptures() const;
+    virtual std::optional<BrokerData> SerializeCaptures() const;
 
     /**
      * Sets the captures frame to one built from *data*.
      *
      * @param data a serialized frame
      */
-    bool DeserializeCaptures(const broker::vector& data);
+    bool DeserializeCaptures(BrokerListView data);
 
     using Func::AddBody;