Add integration length function argument for QA readout

[ysalatheZI]

Description:

When using toolkit function device.qachannels[n].readout.write_integration_weights() sets the integration length to the length of integration weights by default. However, customers want to have the flexibility to set integration length independently.

The expected behavior is that users can set integration length independently.

We add an argument (integration_length) to the function above, and if the user does not configure it, by default the integration_length is equal to the number of integration weights (backwards compatible).

Needs corresponding update in zhinst-utils.

Fixes issue: #

Checklist:

• Add tests for the change to show correct behavior.
• Add or update relevant docs, code and examples.
• Update CHANGELOG.rst with relevant information and add the issue number.
• Add .. versionchanged:: where necessary.

[tobiasah]

Before we merge this, we have to release a new zhinst-utils version that includes the counterpart of this change.
I do not thing that we need backward compatibility here, so apart from this it looks good to me

[tobiasah]

closing this in favor of #274. The change is the same but #274 changes a view other things as well