feat(protocol)

[RohanDisa]

Set up the dataframe interchange protocol scaffolding and implemented the initial XarrayBuffer component.

Introduced a dedicated xarray_sql.dataframe_protocol subpackage so protocol-specific logic has its own organized namespace rather than crowding existing modules. Open to restructuring this if you prefer the classes to live directly under the main package.

Currently placed both the abstract Buffer interface and the concrete XarrayBuffer implementation in core.py; happy to split these into separate files if that aligns better with the project structure.

Once the layout is settled, I’ll proceed with implementing the remaining two protocol classes as discussed.