ref(flags): remove OpenFeatureHook from exports and setup code

[aliu39]

Closes #14645

By un-exporting the Hook, this will simplify setup for users.

The decision to track a single client is something we're doing across all FF integrations. From getsentry/sentry-python#3895:

All provider SDKs so far have the concept of a "client" class, which is used to connect to the server and query flags. Since clients can query multiple projects/environments by specifying a context, it seems rare for an app to use >1 client.
I discussed with @billyvg and we want to stay consistent for all FF integrations, and support flag tracking for only one client, until we get requests for more. Therefore, users should be required to pass a client to the Integration. We'll register hooks on that client, rather than globally.

Updated setup code can be seen at: https://github.com/getsentry/sentry-docs/pull/12222/files
Todo:

• update docstr