Epic: Add support for new endpoints and fields in the Soroban RPC SDK.

[Shaptic]

First, the following endpoints need to be completed:

• Provide getFeeStats as a new endpoint on RPC soroban-rpc#114
• Feature: Expose a getLedgers endpoint soroban-rpc#109
• add getTransactions endpoint soroban-rpc#110

Tasks

Beta Give feedback

1. Add support for new stateChanges in simulateTransaction #936
   objective-2 +1
   
2. Support getFeeStats in the Soroban client
3. Support the new getTransactions endpoint #943
   objective-5 +1
4. Support the new getLedgers endpoint #944
   +0
5. Update the XDR to Protocol 21 #942
   objective-2 +1