Respond to TO-DO regarding timestamp generation during Receipt creation

[severiano-sisneros]

https://github.com/semiotic-ai/timeline_aggregation_protocol/blob/c563878e25e55c05d474cb546a321b4d32dbe3b4/src/receipt.rs#L40

Opening an issue to address the TODO in the code. In my opinion timestamp generation should be included in Receipt generation because the timestamp is specific to the TAP protocol.