[FIX] web: speed up read_progress_bar

The method is used to get progress per column in kanban view
(green-yellow-red-red lines in Project, CRM etc).  There are two main
usages:

1. get statistics for ``kanban_state`` (red/green circles)
2. get statistics for ``activity_state`` (colored clock icon for overdue/today/planned)

Before this commit all cases were handled by calling search_read and then
counting records per group in a python script.  This is very inefficient,
especially for ``activity_state``.

This new implementation relies on ``read_group`` when possible, i.e.,
when both grouping fields (kanban column and progressbar field) are
stored (case 1).  It then falls back on a naive implementation inside
``_read_progress_bar``.  Cases like 2 above can be addressed by
overriding ``_read_progress_bar``.

We also added some minimal test to ensure that we don't break anything.

1. Performance test on 60 K project.task records (kanban_state):

With a filter for 6 records:

```
| measurement        | before | after |
|--------------------+--------+-------|
| number of queries  |      8 |     5 |
| query time, ms     |     11 |     7 |
| remaining time, ms |     21 |     9 |
```

All records:
```
| measurement        | before | after |
|--------------------+--------+-------|
| number of queries  |     67 |     5 |
| query time, ms     |    300 |    55 |
| remaining time, ms |   1780 |    12 |
```

Co-authored-by: Raphael Collet <rco@odoo.com>

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opw-2346901
task-1915411