Fix NfTokenBuyOffers and NfTokenSellOffers RequestParams/Results

[nkramer44]

I noticed that there were some missing fields in NfTokenBuyOffersRequestParams, NfTokenBuyOffersResult, NfTokenSellOffersRequestParams and NfTokenSellOfferResult.

• Both request params classes were missing a LedgerSpecifier field. I defaulted both of them to VALIDATED to keep the existing behavior
• Both result classes were missing limit and marker fields, which made it impossible to page through results.
• BuyOffer and SellOffer both had Amount and Flags as the @JsonProperty field names, but the actual json uses amount and flags. This wasn't actually an issue because Jackson seems to handle the wrong casing, but should be fixed regardless

[codecov]

Codecov Report

Merging #465 (463b6a9) into main (7d72fdd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #465   +/-   ##
=========================================
  Coverage     90.46%   90.46%           
- Complexity     1468     1470    +2     
=========================================
  Files           287      287           
  Lines          4174     4176    +2     
  Branches        339      339           
=========================================
+ Hits           3776     3778    +2     
  Misses          277      277           
  Partials        121      121           

Files Changed	Coverage Δ
...ava/org/xrpl/xrpl4j/model/client/nft/BuyOffer.java	100.00% <ø> (ø)
...pl/xrpl4j/model/client/nft/NftBuyOffersResult.java	100.00% <ø> (ø)
...l/xrpl4j/model/client/nft/NftSellOffersResult.java	100.00% <ø> (ø)
...va/org/xrpl/xrpl4j/model/client/nft/SellOffer.java	100.00% <ø> (ø)
...4j/model/client/nft/NftBuyOffersRequestParams.java	100.00% <100.00%> (ø)
...j/model/client/nft/NftSellOffersRequestParams.java	100.00% <100.00%> (ø)

[sappenin]

LGTM!