WebhookConfig proto for ephemeral URL suffix

[mwaskom]

Describe your changes

Pulling the proto part out of #3660

Checklists

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Compatibility checklist

Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Note

Adds WebhookConfig.ephemeral_suffix for ephemeral app URLs and clarifies requested_suffix as the user-supplied label.

• Proto: modal_proto/api.proto
  • WebhookConfig:
    • Add string ephemeral_suffix = 11 for additional URL suffix in ephemeral Apps.
    • Document requested_suffix as the user-supplied label component of the URL.

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[mwaskom]

Was approved as part of #3660

[mwaskom]

@prbot approve

[modal-pr-review-automation]

Approved 👍. @freider will follow-up review this.

[freider]

lgtm