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