Allow time range filters with fixed start date but open end date in discover queries

### Problem Statement

A discover query can have a time range filter applied. In the filter one could specify both start and end date, or only end date - by applying "last X days" filter. However, when you run experiments it's very convenient being able to monitor their tech metrics in Sentry. The problem is that running experiments have fixed launch date, but open end date.

This can be worked around by putting something like `timestamp:>experiment_start_date` into the query itself. But this approach clutters the query and is not consistent with other time filter scenarios.

### Solution Brainstorm

Add another time range filter, which would let to specify a start date, but leave the end date open and basically equal to "now".

### Product Area

Unknown

No files, tests, or entry points are named. Start by locating the Discover query time-range filter and its query construction, then trace how a fixed start date and an open end date are represented. Done means users can set a start date with the end implicitly equal to now without adding a timestamp condition to the query.

Written by the indexing model from the issue text.