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.

include/xrpl/json/json_value.h

@@ -381,6 +381,9 @@ public:
     /// Return true if the object has a member named key.
     bool
     isMember(std::string const& key) const;
+    /// Return true if the object has a member named key.
+    bool
+    isMember(StaticString const& key) const;
 
     /// \brief Return a list of the member names.
     ///