Add Attachment.Timestamp to the schema

[gasparnagy]

🤔 What's the problem you're trying to solve?

During test and step executions the execution might produce multiple attachments. For longer steps the exact timing of when the attachments were captured might be important.
Currently the Attachment envelop type does not have a timestamp and therefore this information cannot be captured and used.

✨ What's your proposed solution?

Add a timestamp field for the Attachment type just like in any other execution-related envelope types, like TestStepStarted.

⛏ Have you considered any alternatives or workarounds?

As the Attachment type does not have any extensibility point to capture additional information, I don't see any meaningful workaround.
The timestamp can be set as optional for backwards compatibility.

📚 Any additional context?

Needed to properly capture attachments for Reqnroll.

//CC @clrudolphi

[clrudolphi]

Would #301 address your concern?
I doubt it as the timestamp is the best way to sequence messages in the stream to retain a 'natural' feel to them.

[davidjgoss]

Makes sense, I’m in support.