Put PSPDFKit on Gradle's 'api' configuration

[davidschreiber]

This allows users to use PSPDFKit classes in their app's native extension sources as it was possible when we still used the compile configuration.

Details

With our update to PSPDFKit 6.1.1 we changed the source configuration used for PSPDFKit from compile (which is deprecated) to implementation. This had the side-effect that PSPDFKit Java classes would no longer be available in apps that used native Java code. Putting the dependency on the api configuration fixes this issue.

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, samples/Catalog/package.json, and samples/NativeCatalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).