…elastic#75487)

In addition to a route for listing Credentials, this also adds a
utility function which helps create API routes which simply proxy
the App Search API.

The reasoning for this is as follows;
1. Creating new routes takes less effort and cognitive load if we
can simply just create proxy routes that use the APIs as is.
2. It keeps the App Search API as the source of truth. All logic is
implemented in the underlying API.
3. It makes unit testing routes much simpler. We do not need to verify
any connectivity to the underlying App Search API, because that is
already tested as part of the utility.