Proposal: update Nsmf_EventExposure models and add Nupf_EventExposure client

Hello free5GC maintainers,

Thank you for maintaining the project and for reviewing community contributions. We would appreciate your guidance on the preferred approach for adding the OpenAPI support described below.

## Background

We are preparing an SMF contribution for Event Exposure subscriptions. The intended flow is:

1. An NF service consumer creates an `Nsmf_EventExposure` subscription.
2. SMF resolves the corresponding PDU session and selected UPF.
3. SMF creates the related subscription through `Nupf_EventExposure`.
4. The UPF sends event notifications directly to the notification URI supplied by the consumer.

Before implementing this in SMF, we would like to confirm the preferred ownership and generation process for the required OpenAPI models and client.

## Observed gaps

The current `free5gc/openapi` module includes the generated `smf/EventExposure` client, but the available Nsmf models do not appear to include the fields required for UPF event subscriptions, including:

- `UPF_EVENT`
- `upfEvents`
- `bundledEventNotifyUri`

We also could not find a generated `Nupf_EventExposure` client, such as an `upf/EventExposure` package.

## Specification references

Our current reference specifications are:

- 3GPP TS 29.508 V19.4.0
- 3GPP TS 29.564 V19.5.0
- 3GPP TS 29.571 V19.5.0

We understand that free5GC currently primarily follows a Release 17 OpenAPI baseline, so we would like to confirm which specification revision should be used for this contribution.

## Questions

1. Would you recommend submitting and merging a `free5gc/openapi` PR for the required Nsmf models and Nupf client before submitting the corresponding `free5gc/smf` PR?
2. Alternatively, would you prefer the OpenAPI and SMF PRs to be opened in parallel, with the SMF PR explicitly depending on the OpenAPI PR?
3. Should this work follow the current Release 17 baseline, or may it introduce the required Release 19 Event Exposure schemas?
4. Which 3GPP OpenAPI source and OpenAPI Generator version should contributors use?
5. Where should the Nupf-specific models and generated client be placed?
6. Is there an existing generation script or workflow that contributors should follow?
7. If a generated Nupf client is not currently planned, would a minimal handwritten client in the SMF consumer layer be acceptable?

Thank you for your time and guidance. We would be happy to adjust the proposed scope and contribution order based on the maintainers' recommendations.