PARQUET-113: Add specs for LIST and MAP annotations.

Draft specs for using `MAP` and `LIST` annotations.

Please help verify that this can read all existing map and list data correctly!
With the specification as written, there is no value to adding the MAP_KEY_VALUE or LIST_ELEMENT annotations. These would always annotate the repeated group, just like MAP_KEY_VALUE currently does. LIST_ELEMENT would have been useful to differentiate between a single-element group that contains the element type or a single-element group that is the element type, but since it was never used, it doesn't make sense to add it now, when the data it would help for has already been written.