Rework instantiation to be more useful and consistent with the spec

[nyurik]

There are two usecases for this lib -- with and without defaults:

• A typically use case with defaults -- a client code that gets a JSON, resolves maxzoom to 30 if missing, and takes actions like loading tiles.
• Without defaults -- a tile server needs to create and send a minimal tilejson, e.g. if it doesn't support grids value, it shouldn't set it to the default [ ], but instead not set it at all.

Current API actually does the opposite -- the client use-case" serde_json::from_str(str) ignores tilespec defaults, while the server use case with TileJSON::new() sets the defaults.

Also, per specification, several fields must be present from the start, so it would make sense to introduce them as arguments:

"tilejson": "3.0.0",
"tiles": [
    "http://localhost:8888/admin/1.0.0/world-light,broadband/{z}/{x}/{y}.mvt"
  ],

When instantiating, tilejson could still be defaulted to the current version, i.e. 3.0.0. The tiles on the other hand MUST be a non-empty array.

Proposed API (breaking)

impl TileJSONBuilder {
  /// create a builder with tilejson = 3.0.0 and tiles = [ source ]
  fn new(source: String) -> Self {}
  /// create a builder with custom version and possibly multiple sources.
  /// If version is None, will use current default.
  fn new_ext(sources: Vec<String>, version: Option<String>) -> Self {}
  /// generate TileJSON **without** defaults.  Breaking change, so new method name.
  fn build() -> TileJSON {}
  // remove fn tiles(...)
}

impl TileJSON {
  /// consume to ensure each optional value has a proper default per spec. Name is TBD
  fn with_defaults(self) -> Self {}
}

Note that "vector_layers" is required conditionally - depending on the type of the tiles, so in our case we could treat it as optional unless we want to perform validation.

cc: @stepankuzmin @maxammann @Drabble @ka7eh