Price Oracle (XLS-47d): (#4789) (#4789)

Implement native support for Price Oracles.

 A Price Oracle is used to bring real-world data, such as market prices,
 onto the blockchain, enabling dApps to access and utilize information
 that resides outside the blockchain.

 Add Price Oracle functionality:
 - OracleSet: create or update the Oracle object
 - OracleDelete: delete the Oracle object

 To support this functionality add:
 - New RPC method, `get_aggregate_price`, to calculate aggregate price for a token pair of the specified oracles
 - `ltOracle` object

 The `ltOracle` object maintains:
 - Oracle Owner's account
 - Oracle's metadata
 - Up to ten token pairs with the scaled price
 - The last update time the token pairs were updated

 Add Oracle unit-tests

src/ripple/protocol/impl/InnerObjectFormats.cpp

@@ -138,6 +138,15 @@ InnerObjectFormats::InnerObjectFormats()
         {
             {sfAccount, soeREQUIRED},
         });
+
+    add(sfPriceData.jsonName.c_str(),
+        sfPriceData.getCode(),
+        {
+            {sfBaseAsset, soeREQUIRED},
+            {sfQuoteAsset, soeREQUIRED},
+            {sfAssetPrice, soeOPTIONAL},
+            {sfScale, soeDEFAULT},
+        });
 }
 
 InnerObjectFormats const&