TTL: allow custom start time

## Feature Request

**Is your feature request related to a problem? Please describe:**
For some large tables, I would like to run nightly job, once per day, during off peak hours. I am able to set `TTL_JOB_INTERVAL=24h` but have not way to configure when the TTL job should start.

**Describe the feature you'd like:**
The ability to specify approximate time when TTL job should start, for example TTL_JOB_INTERVAL=24h `TTL_JOB_START='02h00'`

**Describe alternatives you've considered:**
I considered directly manipulating internal TiDB tables, but this is not supported and I am not 100% sure how the whole thing works. There might be caching or other internal mechanisms that I'm not aware of. I don't want to risk breaking TTL or causing inconsistencies.
```
TiDB root@127.0.0.1:mysql> SELECT NOW(); ALTER TABLE test.modal_poll_entry TTL=`created` + INTERVAL 30 DAY TTL_JOB_INTERVAL='10m';
You're about to run a destructive command.
Do you want to proceed? (y/n): y
Your call!
+---------------------+
| NOW() |
+---------------------+
| 2025-06-25 18:22:18 |
+---------------------+
1 row in set

Query OK, 0 rows affected

TiDB root@127.0.0.1:mysql> select * from tidb_timers\G
***************************[ 1. row ]***************************
ID | 1
NAMESPACE | default
TIMER_KEY | /tidb/ttl/physical_table/110/110
TIMER_DATA | {"table_id":110,"physical_id":110}
TIMEZONE |
SCHED_POLICY_TYPE | INTERVAL
SCHED_POLICY_EXPR | 10m
HOOK_CLASS | tidb.ttl
WATERMARK | 2025-06-25 17:53:41
ENABLE | 1
TIMER_EXT | {"tags": ["db=test", "table=modal_poll_entry"]}
EVENT_STATUS | IDLE
EVENT_ID |
EVENT_DATA |
EVENT_START |
SUMMARY_DATA | {"last_job_request_id":"a6e8ea23dd3b4a5999b461e3819a07f0","last_job_summary":{"total_rows":0,"success_rows":0,"error_rows":0,"total_scan_task":1,"scheduled_scan_task":1,"finished_scan_task":1}}
CREATE_TIME | 2025-06-25 17:53:40
UPDATE_TIME | 2025-06-25 18:22:19
VERSION | 7
1 row in set
TiDB root@127.0.0.1:mysql> UPDATE mysql.tidb_timers SET UPDATE_TIME='2025-06-25 18:23:00', VERSION=8 WHERE ID=1;
Query OK, 1 row affected
TiDB root@127.0.0.1:mysql> select * from tidb_timers\G
***************************[ 1. row ]***************************
ID | 1
NAMESPACE | default
TIMER_KEY | /tidb/ttl/physical_table/110/110
TIMER_DATA | {"table_id":110,"physical_id":110}
TIMEZONE |
SCHED_POLICY_TYPE | INTERVAL
SCHED_POLICY_EXPR | 10m
HOOK_CLASS | tidb.ttl
WATERMARK | 2025-06-25 18:22:20
ENABLE | 1
TIMER_EXT | {"tags": ["db=test", "table=modal_poll_entry"]}
EVENT_STATUS | IDLE
EVENT_ID |
EVENT_DATA |
EVENT_START |
SUMMARY_DATA | {"last_job_request_id":"55ca210912c8472b9b726d37c309b600","last_job_summary":{"total_rows":0,"success_rows":0,"error_rows":0,"total_scan_task":1,"scheduled_scan_task":1,"finished_scan_task":1}}
CREATE_TIME | 2025-06-25 17:53:40
UPDATE_TIME | 2025-06-25 18:23:00
VERSION | 8
1 row in set
```

**Teachability, Documentation, Adoption, Migration Strategy:**
This would allow tables that have heavy TTL jobs to run during the night, when workload is low.