pageserver: optimize heatmap downloads with S3 conditional GET

[jcsp]

In #6123, periodic download of heatmaps is added for secondary mode locations.

While downloading the heatmap is not bandwidth intensive, we can avoid transferring any body at all if the heatmap has not changed: https://docs.aws.amazon.com/AmazonS3/latest/API/API_GetObject.html (If-None-Match or If-Modified-Since)