Ensure correcty tying of API key's restrictIndices parameter

The `restrictIndices` of API keys should be typed as:

- an array of strings OR
- a comma-separated string

Both format should be handled at serialization/deserialization time.

Start by locating the API-key model and the serialization/deserialization entry points in the Java client. Verify that both arrays of strings and comma-separated strings are handled for restrictIndices, then add or update coverage for both representations.

Written by the indexing model from the issue text.