Epic: resolve bugs related to CEL filtering and add missing filters

[avinal]

This epic contains bugs or missing features in current filtering. After merging of PR #404 many bugs related to inconsistencies in filtering and ordering were fixed, but many older filters are missing, and few bugs were introduced. This task list tracks them and the progress in resolving them. You are welcome to create a new issue from the task list and work on it.

Bugs

• [x] #498
• [ ] duration function is unavailable
• [x] https://github.com/tektoncd/results/issues/620

Missing

• [ ] #516

Misc

• [x] Add documentation for cel2sql filters #566