feat(functions): configure RPC and HTTP invocation modes

Follow-up to Kong/volcano-hosting#908.

Add CLI support for function invocation metadata:

- accept `invocation_mode` (`rpc` or `http`) on function create/update
- accept `http_auth_mode` (`volcano` or `none`)
- accept an OpenAPI 3.0/3.1 JSON file and support clearing it
- require an explicit public-function choice before allowing `http_auth_mode=none`
- display mode/auth in function get/list output without printing full OpenAPI documents in list views
- add generated-client and command tests
- document a webhook deployment example

The hosting API remains backward compatible: omitted fields default to RPC + Volcano auth.

Start with the function create, update, get, and list CLI entry points, then inspect the generated client and command tests mentioned in the issue. Verify invocation and authentication options, OpenAPI file setting and clearing, public-function validation, backward-compatible defaults, and output behavior. Done includes the generated-client and command tests plus a documented webhook deployment example.

Written by the indexing model from the issue text.