Provide a conversion function from icinga::String to boost::string_view

Boost.Beast changed the signature of
boost::beast::http::basic_fields::set in version 1.74 so that no longer
allows passing an icinga::String instance as value. This adds a
conversion function so that it works again.

lib/base/string.cpp

@@ -127,6 +127,18 @@ String::operator const std::string&() const
 	return m_Data;
 }
 
+/**
+ * Conversion function to boost::string_view.
+ *
+ * This allows using String as the value for HTTP headers in boost::beast::http::basic_fields::set.
+ *
+ * @return A boost::string_view representing this string.
+ */
+String::operator boost::string_view() const
+{
+	return boost::string_view(m_Data);
+}
+
 const char *String::CStr() const
 {
 	return m_Data.c_str();

lib/base/string.hpp

@@ -6,6 +6,7 @@
 #include "base/i2-base.hpp"
 #include "base/object.hpp"
 #include <boost/range/iterator.hpp>
+#include <boost/utility/string_view.hpp>
 #include <string>
 #include <iosfwd>
 
@@ -71,6 +72,7 @@ public:
 	bool operator<(const String& rhs) const;
 
 	operator const std::string&() const;
+	operator boost::string_view() const;
 
 	const char *CStr() const;