Fetches dynamic dropdown / search options for a workflow app action by
invoking the app version with is_filling_dynamic_dropdown set.

Use this when listWorkflowStepTypes detail=full shows a field with
dynamic or search metadata (e.g. Slack channel uses
list_channels.id). Pass the perform key as action_key, the app's
app_version_id, a credential_id, and the app_action_type (search
or create) that contains that action_key.

Example — list Slack channels the bot has joined:

POST /v1/workflow-apps/{slack_app_id}/actions/list_channels/input-options
{ "app_version_id": "...", "app_action_type": "search", "credential_id": "..." }

Then set the chosen option's value on the step's operation.perform.inputs.

action_key is the perform key from the app definition (create or search).
Callers must supply app_version_id from the public catalog field
(app_version_id / published_app_version_id) or from existing step config.

OAuth callers require workflows:read for app_action_type=search, and
workflows:update for app_action_type=create.