Add support for returning sync token values after committing transactions.

[AhmedZaki99]

Fixes #29

Changes include:

• Update the SyncResponse model with a new property SyncToken that maps to sync_token Json property on the transaction response.
• Update the IAdvancedTodoistClient.ExecuteCommandsAsync and its implementation in TodoistClient to return Task<string> containing the sync_token included in the response instead of plain Task.
• Update the ITransaction.CommitAsync() and its implementation in Transaction to return Task<string> following the change made in IAdvancedTodoistClient.ExecuteCommandsAsync.
• Add assertion in TransactionTests.CreateProjectAndCreateNote_Success() test method to check the sync_token value returned by the ITransaction.CommitAsync() method.

[olsh]

Thank you for your contribution! 👍🏻