Fix NFT Amounts

[nkramer44]

Fixes #450

Some of the amount fields in NFT related objects were typed as XrpCurrencyAmounts, when they should have been CurrencyAmount. This was noticed because parsing transaction metadata with an AffectedNode for NFTokenOffer objects would throw an exception. This is because xrpl4j could not deserialize to a MetaNfTokenOfferObject because the amount field could not be deserialized correctly.

This PR changes the amount field in SellOffer, BuyOffer, NfTokenOfferObject and MetaNfTokenOfferObject from XrpCurrencyAmount to CurrencyAmount.

[codecov]

Codecov Report

Merging #451 (f91a30a) into main (1da875a) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #451   +/-   ##
=========================================
  Coverage     90.43%   90.43%           
  Complexity     1464     1464           
=========================================
  Files           285      285           
  Lines          4163     4163           
  Branches        339      339           
=========================================
  Hits           3765     3765           
  Misses          277      277           
  Partials        121      121           

Impacted Files	Coverage Δ
...ava/org/xrpl/xrpl4j/model/client/nft/BuyOffer.java	100.00% <ø> (ø)
...va/org/xrpl/xrpl4j/model/client/nft/SellOffer.java	100.00% <ø> (ø)
...g/xrpl/xrpl4j/model/ledger/NfTokenOfferObject.java	100.00% <ø> (ø)