Add NFTokenPages to account_objects RPC: (#4352)

- Include NFTokenPages in account_objects to make it easier to
  understand an account's Owner Reserve and simplify app development.
- Update related tests and documentation.
- Fix #4347.

For info about the Owner Reserve, see https://xrpl.org/reserves.html

---------

Co-authored-by: Scott Schurr <scott@ripple.com>
Co-authored-by: Ed Hennis <ed@ripple.com>

src/test/rpc/AccountLinesRPC_test.cpp

@@ -679,6 +679,7 @@ public:
             auto aliceLines = getNextLine(env, alice, std::nullopt);
             constexpr std::size_t expectedIterations = 16;
             constexpr std::size_t expectedLines = 2;
+            constexpr std::size_t expectedNFTs = 1;
             std::size_t foundLines = 0;
 
             auto hasMarker = [](auto const& aliceLines) {
@@ -729,7 +730,7 @@ public:
             // this test will need to be updated.
             BEAST_EXPECT(
                 aliceObjects[jss::result][jss::account_objects].size() ==
-                iterations);
+                iterations + expectedNFTs);
             // If ledger object association ever changes, for whatever
             // reason, this test will need to be updated.
             BEAST_EXPECTS(