Add new hook HookLoadFileExtended that allows plugins to supply Zeek script code to parse.

The new hooks works similar to the existing `HookLoadFile` but,
additionally, allows the plugin to return a string that contains the
code to be used for the file being loaded. If the plugin does so, the
content of any actual file on disk will be ignored (in fact, there
doesn't even need to be a file on disk in that case). This works for
both Zeek scripts and signatures.

There's a new test that covers the new functionality, testing loading
both scripts and signatures from memory. I also manually tested that the
debugger integration works, but I don't see much of a way to add a
regression test for that part.

We keep the existing hook as well for backwards compatibility. We could
decide to deprecate it, but not sure that buys us much, so left that
out.

Closes #1757.

src/PolicyFile.h

@@ -14,12 +14,16 @@
 // policy_filename arguments should be absolute or relative paths;
 // no expansion is done.
 
+#include <optional>
+#include <string>
+
 namespace zeek::detail
 	{
 
 int how_many_lines_in(const char* policy_filename);
 
-bool LoadPolicyFileText(const char* policy_filename);
+bool LoadPolicyFileText(const char* policy_filename,
+                        const std::optional<std::string>& preloaded_content = {});
 
 // start_line is 1-based (the intuitive way)
 bool PrintLines(const char* policy_filename, unsigned int start_line, unsigned int how_many_lines,