• alternative to feat: Add support for to use a object with a union type #580
• required for feat: Add union type to support the ClientOptions class in the client constructor gapic-generator-php#725

I see two ways to support ClientOptions for ClientOptionsTrait::buildClientOptions:

1. Call toArray on ClientOptions and keep the logic the same (what we have in this PR)

   • simplest way
   • means that we convert options passed in as ClientOptions to an array and then back again (not so good)

2. Refactor this class to have a new method which takes ClientOptions, and remove all the backwards compatibility stuff

   • this would be best for the long-run
   • would require a bit more work

TODO: prototype option 2 to see if it's feasible