- Fetch and compile API specifications to ApiModels immediately on load

- The extension version of API Explorer now calls `versions` retrieve specs, and calls `api_spec` to load and compile them

- The standalone version still bundles the specs, but the loader now compiles them to ApiModels and clears the bundled spec content to reduce the memory footprint

- This will only work in Looker 21.3 and above (after the Looker PR introducing `api_spec` is merged) so the Extension now requires Looker 21.3 or above

- Omit legacy and deprecated specifications (API 3.0 for example)

- Updated readme for extension manifest

- TODO implement fetching and compiling specs on demand when selected via the UI