Improve handling of latest extension versions in spec

Multiple TODOs are still open in the `ExtensionSpec` template. These need to be resolved be generally improving the handling of latest versions of extensions.

- [ ] The spec will need to be adapted to be informed by https://github.com/ampproject/amphtml/blob/main/build-system/compile/bundles.config.extensions.json, where the latestVersion is stored with the spec (and the highest version is not always the same as latest), as well as any additional Bento metadata.
- [ ] There is one case where an extension lacks a `'latest'` version, but maybe not for long. See https://github.com/ampproject/amphtml/pull/34636.
- [ ] Why isn't the sorting done at ingestion time? Then `getLatestVersion()` could just be doing something like `return self::LATEST_VERSION`.

https://github.com/ampproject/amp-toolbox-php/blob/c680d60e9cf03c0e4c8a0f0ff4b3b3c662058eea/bin/src/Validator/SpecGenerator/Template/ExtensionSpec.php#L29-L67