Add intervals for scheduled task

[so-saf]

Hi!

Following up on PR #399, which I left unfinished half a year ago:

In this PR, I want to add intervals for scheduled tasks.

List of changes:

• Added a new interval field to ScheduledTask.
• Implemented the SchedulerLoop class—an abstraction over the scheduler loop.
• Added a new loop_interval argument to SchedulerArgs.
• Moved the skip_first_run mechanism into SchedulerLoop.run().
• The teskiq.api.scheduler.run_scheduler_tasks function now uses SchedulerLoop.
• Implemented the schedule_by_interval method in the AsyncTaskiqDecoratedTask and AsyncKicker classes.
• Wrote tests for the new logic.
• Updated the documentation.

The main idea is to iterate through all tasks every loop_interval and check if they need to be executed.

Example:

@broker.task(schedule=[{"interval": 5, "args": [1]}])  
async def add_one(value: int) -> int:  
    await asyncio.sleep(0.5)  
    return value + 1  

Logs:

[2025-08-18 11:07:50,547][INFO   ][run:run_scheduler:396] Starting scheduler.
[2025-08-18 11:07:50,551][INFO   ][run:run_scheduler:398] Startup completed.
[2025-08-18 11:07:51,001][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.
[2025-08-18 11:07:56,002][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.
[2025-08-18 11:08:01,002][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.
[2025-08-18 11:08:06,002][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.
[2025-08-18 11:08:11,001][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.
[2025-08-18 11:08:16,001][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.
[2025-08-18 11:08:21,001][INFO   ][run:send:160] Sending task test:add_one with schedule_id 5b32c74a07d447d68f506b089616d526.

[so-saf]

I saw that several tests failed on older versions of Pydantic and Python, so I pushed fixes.

[so-saf]

I have resolved the merge conflicts and fixed the failing tests.