[Traceql] Add support for event timestamp

[ie-pham]

What this PR does: Add Traceql support for event timestamp. User will be able to query for events by how much time as elapsed since span start time to the timestamp of the event using event:timeSinceStart.

Example:
{ event:timeSinceStart > 1m }
This query can help find spans where an event took place at least 1 minute after the span start time.

Which issue(s) this PR fixes:
Fixes #

Checklist

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

[ie-pham]

This wasn't part of the traceql addition doc but as I was debugging something, I thought this could be a very useful query.

[joe-elliott]

Love this PR.

Can we get a test in ./tempodb/tempodb_search_test.go : TestSearchCompleteBlock ?

[knylander-grafana]

Thank you for adding some doc :)