Add ETag support for those REST API calls that do not expand macros.

Use case:

• We have pages that are reset to an updated state on a daily basis.
• In the page history, we do not want to see the idempotent updates.
• However, we want to overwrite edits made by others (persons or scripts); i.e. the script cannot skip the update if the source data has been changed.
  We want to send an alert to the page maintainer if this happens.
• Retrieving the data from Confluence to do an "is this unchanged" check before updating risks a race condition. Also, we want to avoid triggering a flurry of alerts whenever a Confluence update returns more or different information.