Configurable batch sizes with the max_elements subscription option

[MrAnno]

This PR adds the max_elements option to subscription configurations, making it possible to control the maximum number of events the client aggregates.

For example, setting it to 1 allows real-time event flow, which makes debugging or the validation of an initial OpenWEC setup easier.

Config example:

# conf/my-test-subscription.toml
[options]
max_elements = 1

Implementation/testing notes:

• I tested the database migration only with SQLite.
• I haven't added the new option to the deprecated CLI interface.
• I haven't added the new option to the v1 version of ImportExport (I'm not sure if this is right or not).

[vruello]

Thanks for this nice PR 👍

I need some time to review and test it, but it looks pretty complete at first sight.

[vruello]

I have added a check for max_elements in database tests. I also tested the feature on my lab, it works as expected 👍

I haven't added the new option to the v1 version of ImportExport (I'm not sure if this is right or not).

Since the default value of max_elements is None, there is no need to change the v1 import function. In addition, this change does not invalidate the previously generated v2 export files, so there is no need for a v3: editing the v2 schema as you did is fine.

Thank you very much!

[MrAnno]

Thank you :)