Move service methods for paginated endpoints to use PageCollection pattern

[annelo-msft]

Confirm this is not a feature request for the underlying OpenAI API.

• This is not a feature request for the underlying OpenAI API

Confirm this is not a feature request for Azure OpenAI.

• This is not a feature request for Azure OpenAI

Describe the feature or improvement you're requesting

The following service endpoints return paginated lists of results and should move to the SCM-based client PageCollection pattern:

• List fine-tuning jobs
• List fine-tuning events
• List fine-tuning checkpoints
• List batch

The pattern is illustrated for the list assistants and list vector stores endpoints in this PR.

Additional context

No response