Update InvariantCheck, LoanManage, and LoanPay to make tests work

- Primarily updating tests, and fixing stuff that didn't work.
- Tests still not expected to pass.
- Add Json::Value::isMember(StaticString) so SFields can be used.
- Validate more fields in ValidLoan Invariant

include/xrpl/json/json_value.h

@@ -395,6 +395,9 @@ 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.
     ///

include/xrpl/protocol/detail/ledger_entries.macro

@@ -573,12 +573,13 @@ LEDGER_ENTRY(ltLOAN, 0x0089, Loan, loan, ({
     //   represents interest owed to the vault.
     // There are two additional values that can be computed from these:
     // - InterestOutstanding: TotalValueOutstanding - PrincipalOutstanding
-    //   This is the total amount of interest still pending on the loan,
+    //   The total amount of interest still pending on the loan,
     //   independent of management fees.
     // - ManagementFeeOwed: InterestOutstanding - InterestOwed
-    //   This is the amount of the total interest that will be sent to the
+    //   The amount of the total interest that will be sent to the
     //   broker as management fees.
     {sfPrincipalOutstanding, soeDEFAULT},
+    {sfReferencePrincipal, soeDEFAULT},
     {sfTotalValueOutstanding, soeDEFAULT},
     {sfInterestOwed,         soeDEFAULT},
     // Based on the original principal borrowed, used for