feat(functions): configure RPC and HTTP invocation modes

[marckong]

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.