mgr/rbd_support: Fix "start-time" arg behavior

mgr/rbd_support: Fix "start-time" arg behavior

The "start-time" argument, optionally passed when adding or removing an
mirror image snapshot schedule or a trash purge schedule, does not
behave as intended. It is meant to schedule an initial operation at a
specific time of day in a given time zone. Instead, it offsets the
schedule’s anchor time. By default, the scheduler uses the UNIX epoch as
the anchor to calculate recurring schedule times, and "start-time"
simply shifts this anchor away from UTC, which can confuse users. For
example:

```
$ # current time
$ date --universal
Wed Dec 10 05:55:21 PM UTC 2025
$ rbd mirror snapshot schedule add -p data --image img1 1h 19:00Z
$ rbd mirror snapshot schedule ls -p data --image img1
every 15m starting at 19:00:00+00:00
```

A user might assume that the scheduler will run the first snapshot each
day at 19:00 UTC and then run snapshots every 15 minutes. Instead, the
scheduler runs the first snapshot at 18:00 UTC and then continues at the
configured interval:

```
$ rbd mirror snapshot schedule status -p data --image img1
SCHEDULE TIME        IMAGE
2025-12-10 18:00:00  data/img1
```

Additionally, the "start-time" argument accepts a full ISO 8601
timestamp but silently ignores everything except hour, minute, and time
zone. Even time zone handling is incorrect: specifying "23:00-01:00"
with an interval of "1d" results in a snapshot taken once per day at
22:00 UTC rather than 00:00 UTC, because only utcoffset.seconds is used
while utcoffset.days is ignored.

Fix:
Similar to the handling of the "start" argument in the FS snap-schedule
manager module, require "start-time" to use an ISO 8601 date-time format
with a mandatory date component. Time and time zone are optional and
default to 00:00 and UTC respectively.

The "start-time" now defines the anchor time used to compute recurring
schedule times. The default anchor remains the UNIX epoch. Existing
on-disk schedules with legacy-format "start-time" values are updated to
include the date Jan 1, 1970.

The `snap schedule ls` output now displays "start-time" with date and
time in the format "%Y-%m-%d %H:%M:00". The display time is in UTC.

Fixes: https://tracker.ceph.com/issues/74192
Signed-off-by: Ramana Raja <rraja@redhat.com>