Investigate version-specific validation steps

[javagl]

The current implementation of the validator is focussed on 3D Tiles 1.1. As suggested by the semantic versioning, version 1.1 mainly consisted of additions to the existing standard, and these additional elements are covered in the validation. Beyond that, the asset.version is largely ignored. Additional checks should be added that take these version-specific differences into account. This means that certain elements (like implicit tilesets) are not valid in a 3D Tiles 1.0 tileset, and a tileset that has an asset.version of 1.0 but contains implicit tiling information should be considered to be invalid.

This is related to #231 , which specifically refers to the question of which extensions are allowed or expected in certain tileset versions.

[javagl]

One specific difference between 1.0 and 1.1 that affects the validator:

• in version 1.0, the tile.children was allowed to be an empty array
• in version 1.1, the tile.children has the requirement of minItems: 1 in the schema.

The latter is actually an inconsistency in the spec, tracked at CesiumGS/3d-tiles#752 - depending on how this is resolved, we may consider a special treatment for this case in the validator.