Horizon v2.31.0 and RPC v21.4.0 updates

Updates for Horizon v2.31.0
(see: #100)

1. The new Horizon API endpoint: POST /transactions_async has been added.
   It is now covered in the sdk by following new methods:

sdk.submitAsyncTransaction()
sdk.submitAsyncFeeBumpTransaction()
sdk.submitAsyncTransactionEnvelopeXdrBase64()

see also: transaction_async_test.dart

2. Error schema change: POST /transactions now includes a the hash in its timeout response:

When submitting a transaction to the network, the sdk can throw the exception SubmitTransactionTimeoutResponseException
This exception now contains the hash of the transaction if available.

Updates for RPC v21.4.0
(see: #100)

• added support for the new endpoints: getTransactions, getFeeStats and getVersionInfo.
• fixed getEvents pagination options

Update SEP-12 support

• added the new transaction_idrequest parameter to get and put customer requests.

Update SEP support for web

The sdk now allows developers to set custom request headers for the SEP requests.
Although the sdk does not yet offer full support for web, this is a step that brings us closer to it,
because it helps us to avoid CORS issues.

Headers and Web fixes

Fixes:

• fix request headers handling (see #98)
• fix sequence number handling for web (see #97)

Breaking changes:

• The sequence number has been changed from int to BigInt

Null-Safety improvements

Null-Safety improvements:

With the introduction of null-safety in dart, the sdk was migrated automatically, which resulted in all class member variables becoming nullable. Most places have been adapted in the mealtime, but there are still a few places that need to be updated.
This release updates the remaining places, especially classes representing horizon responses but also different xdr classes.

Big Amounts on Web:

This release also adds support for big amounts on web for many operations such as, change_trust,
create_account, payment, path_payment_strict_send, path_payment_strict_receive, manage_sell_offer,
manage_buy_offer, create_passive_sell_offer, clawback, create_claimable_balance, liquidity_pool_deposit,
liquidity_pool_withdraw. See also: #96

Breaking changes:

ContractCreditedEffectResponse and ContractDebitedEffectResponse moved from liquidity_pools_effects_responses.dart to soroban_effects_responses.dart

LiquidityPoolDepositedEffectResponse: reservesDeposited is now a list of AssetAmount instead of ReserveResponse

LiquidityPoolWithdrewEffectResponse: reservesReceived is now a list of AssetAmount instead of ReserveResponse

LiquidityPoolRevokedEffectResponse: reservesRevoked is now a list of LiquidityPoolClaimableAssetAmount instead of ReserveResponse

TrustLineFlagsUpdatedEffectResponse moved from misc_effects_responses.dart to trustline_effects_responses.dart

CreatePassiveSellOfferOperationResponse: removed offerId since it is always null

LiquidityPoolDepositOperationResponse: reservesMax & reservesDeposited are now a list of AssetAmount instead of ReserveResponse

Operation: id is now a String instead of int

EffectsRequestBuilder forOperation(String operationId): operationIdis now aStringinstead ofint`

Future<OperationResponse> operation(String operationId): operationId is now a String instead of int

Soroban improvements

1. Soroban Server improvements:

• add getAccountand getContractData methods
• add missing fields oldestLedger and latestLedger to the get health response
• add useful xdr conversions: ledgerEntryData and keyVal for LedgerEntry
• add useful xdr conversions: xdrTransactionEnvelope, xdrTransactionResult, xdrTransactionMeta for GetTransactionResponse
• add missing field txHash to EventInfo
• add useful xdr conversion: valueXdr to EventInfo
• improve simulate transaction result: xdr and auth are not nullable any more

2. SEP-11 (TxRep):

• add soroban support

3. XDR helper methods

• multiple useful additional helper methods in the soroban and classic XDR classes.

4. Breaking changes:

• renamed Row to OrderBookRow for the order book response
• typo: renamed statusChangesto stateChanges for the soroban simulate transaction response

Protocol 21 Support

adds support for protocol 21. see #93

Update Soroban RPC Args

• update soroban RPC args for sendTransaction and getTransaction
• allow optional httpClient to init StellarSDK. See #91

SEP-08 support

adds support for SEP-08. See doc.

Update SEP-01 & improvements

• update SEP-01 currency fields: add contract field
• improve streams in the request builders. see: #90
• update the request builder to maintain the original path and query. see: #89

Update SEP-06

• update SEP-09 fields (see #86)
• update SEP-06 to reflect current SEP-06 doc version. See also: SDK doc. Resolves #87

SEP-38 Support

• add sep-38 support see doc