Implement BufferParams in the python binding

[gastmaier]

!important: In case #1344 does not revert iio_buffer_params. In case #1344 does not revert iio_buffer_params.

8291539 ("task: Implement iio_buffer_params struct and make CMA selectable at runtime") implemented iio_buffer_params, replacing idx with a struct. In the python binding, the change broke usage since dropped optional idx with required params. Define the struct in python, defaulting .idx to 0 and .dma_allocator = IIO_DMA_ALLOCATOR_SYSTEM, and make optional, to restore compatibility with legacy code (idx was hardly set).

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PR Checklist

• I have conducted a self-review of my own code changes
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[nunojsa]

In case #1344 does not revert iio_buffer_params.

I'm fairly sure the revert is to be merged 😅

[gastmaier]

still useful to see the shortcoming of removing the default/optional ... and the implementation, since there is another similar TODO in the file

[gastmaier]

Revert was merged, so this is not relevant anymore