This PR adds indigenous lands to the `boundary` layer by adding new area features for these objects, along with generalization up to z4. It borrows heavily from the technical processing chain of the `park` layer.
I added new `class`, `name`, and `name:xx` attributes to the boundary layer, in order to support the new area features. Should we (or someone extending this schema) come up with new objects to place in the boundary layer in the future, `class` would support additional categories.
This PR adds support for `boundary=aboriginal_lands` by adding it to the `park` layer. While these lands are certainly not "parks", they have similar treatment from a technology perspective and can benefit from the existing processing chain established in that layer. I set all of these objects to `class=aboriginal_lands` in the tiles, including at the lowest zoom in order to separate it from the protected area merging implemented in #1160. In order to distinguish these from general parks, I expose the `class` attribute for these objects at z4 and also ensure that the z4 generalization is performed separately for protected areas versus aboriginal lands.
This unblocks #ZeLonewolf/openstreetmap-americana#105, which describes why having indigenous land boundaries is an important general feature on the map. This is also consistent with my suggested implementation in https://github.com/openmaptiles/openmaptiles/issues/1296#issuecomment-967749403.
Brian Sperlongano
