register_factory should be a user-level API

[ekzhu]

What feature would you like to be added?

The register_factory method in the core's AgentRuntime API should be a user-friendly API. This entails the following changes:

1. type should be string or AgentType
2. expected_types should be optional
3. type and agent_factory should be positional arguments
4. doc string

Why is this needed?

BaseAgent.register is not applicable in scenarios where the factory and subscriptions are registered separately with the runtime. For example, when importing a 3rd party agent class implementation, we cannot assume a default subscription on the agent class, and we would like to specify the subscription on our side.