Implement Lending Protocol (unsupported) (#5270)

- Spec: XLS-66
- Introduces amendment "LendingProtocol", but leaves it UNSUPPORTED to
  allow for standalone testing, future development work, and potential
  bug fixes.
- AccountInfo RPC will indicate the type of pseudo-account when
  appropriate.
- Refactors and improves several existing classes and functional areas,
  including Number, STAmount, STObject, json_value, Asset, directory
  handling, View helper functions, and unit test helpers.

src/libxrpl/json/json_value.cpp

@@ -1042,6 +1042,12 @@ Value::isMember(std::string const& key) const
     return isMember(key.c_str());
 }
 
+bool
+Value::isMember(StaticString const& key) const
+{
+    return isMember(key.c_str());
+}
+
 Value::Members
 Value::getMemberNames() const
 {