Indicate expiration of signed url

[philvarner]

A common use case when using STAC API with STAC Assets that reference S3 objects is to to generate presigned URLs on egress. However, these urls have a (usually) pretty short expiration time. I think it would be useful to notate on these that they will expire.

Maybe a field "authentication:expiry" with a RFC3339 datetime of the URL expiration?

I could also see a use for an authentication scheme of a new type none that auth:refs could use, and then that scheme definition indicates that there are no additional authentication steps required for access, but that the href will expire though a field like "expiry".

[m-mohr]

Would the expires flag from the timestamps extension work?