[FEA] Rename `cudf::structs::detail::flattened_table` and its member function `flattened_columns()`

The current names of these class/function are very confusing:
* The class `flattened_table` is not any table. Instead, it is a data structure holding various types of data.
* The function name `flattened_columns()` suggests that the return type is a list of columns, but it returns a `table_view` instead.

With these name, devs may misunderstand their purpose and make wrong assumption. The issue was raised in several places, for example: https://github.com/rapidsai/cudf/pull/12878#discussion_r1126932626, and https://github.com/rapidsai/cudf/pull/12878#discussion_r1128235975.

We should rename those class/function to better reflect their purposes.