[epic] Protocol 20 SDK Support

[mollykarcher]

This epic will be used to track the necessary changes to the SDK/Horizon API pursuant to Soroban / Protocol 20

minimal to support non-breaking compatibility with horizon API:

• Add support for serialization aspects of new Ops, InvokeHostFunctionOp, BumpFootprintExpirationOp, RestoreFootprintOp in api response handlers (protocol definition). Support for new InvokeHostFunctionOperation #472
• Add/Ensure support for contract addresses (i.e. C...).
  • valid in to/from fields in the InvokeHostFunctionOp:AssetBalanceChanges object. Support for new InvokeHostFunctionOperation #472
  • contract/address strkey support - Add strkey support for contract ids #471
• Add support for new asset stats fields: num_contracts and contracts_amount. Support for new stats in AssetResponse #473

additional functionality for soroban specific contract usage(may move to a separate ticket)

• Ability to submit Soroban/Protocol 20 Operations in Tx's, BumpFootprintExpirationOp, InvokeHostFunctionOp, RestoreFootprintOp
  • add support for using soroban rpc to use preflight which is required for the operations. Obtain tx Ext.SorobanData and InvokeHostFunction.Auth and the required miniumFee, to be included in the tx submitted. refer to js sdk implementation of preflight usage in prepareTransactionfor example - https://github.com/stellar/js-soroban-client/blob/main/src/server.ts#L526
• java-stellar-sdk: add support for fees #479
• add support for contract data ScVal conversions to sensible native java types, refer to js sdk's functional implementation of nativetoScVal and scValToNative for example and scope of conversions, i.e. all the different val types.
• add an abstraction such as a pub/sub interface for getting contract events from soroban rpc as a stream.
• add test coverage with mocked rpc interface.
• add support for fetching contract storage key/value data, refer to js sdk implementation of getContractData()

follow the existing go p20/xdr sdk interface conventions.

[sydneynotthecity]

@sreuland to review and make more tickets for missing work

[sreuland]

closing this epic, #490 will be used going forward for tracking.