[FEATURE] Power Grid Meta Data attributes enum

[petersalemink95]

In #648 / #613 enums were added for ComponentType. AttributeType is still lagging: attributes of components are listed only by their strings on the Python side.

This issue proposes to create (an) appropriate enum(s) for AttributeType as well.

There are multiple ways to implement this, as was also stated in this comment: https://github.com/PowerGridModel/power-grid-model/pull/648#discussion_r1658305065. After internal discussions we concluded to start with the general flat list, i.e.:

"display(pd.DataFrame(output_data[ComponentType.line][ComponentAttributes.p_from][0]))"