[FEAT] Add runs count method

[mtricht]

Is your feature request related to a problem? Please describe.
The RunsClient.list method returns a summary of task runs matching a filter. Setting include_payloads=False reduces the response size but it still is quite heavy when I only needs the total number of matching runs. There also seems to be a limit of 20k task runs when using RunsClient.list.

Describe the solution you'd like
I would like to see count and aio_count methods added to the RunsClient. These should accept the same filtering parameters as list but return a simple integer. I suspect this will require a new server-side endpoint and I assume the server has a way of querying this.

Describe alternatives you've considered

Additional context

[mrkaye97]

Hey @mtricht - this is tricky, because if the runs table is large, this generally will result in a sequential scan over potentially tens or hundreds of millions (or even billions) of rows, which will blow up CPU on the database. You might have looked at the various count queries we have elsewhere (mostly internal for pagination + powering the dashboard) and seen that we have an initial CTE that limits the result set to usually 20k rows before doing the count, in order to prevent this sort of behavior.

Would a maximum count, at which point you just know the count is > that value, suffice? Or do you need the exact number even for very large values?