Respect passed headers in read path requests

[mapno]

What this PR does:

#4242 broke compatibility with non-vanilla distributions of Tempo that rely on headers in the read path.

This PR respects headers in queries and forwards them to queriers as before.

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[javiermolinar]

I think the change was ok, we need to identify the missing header (X-Org-id or Authorization) and add it to the prepareRequestForQueriers

[mdisibio]

This change LGTM, lean towards reverting this behavior now to fix things, and we can revisit the alloc savings once we know more.