[rollout] feat: Allow customization of async server class

[ultmaster]

What does this PR do?

This PR contains two aspects:

1. Introduction of a new configuration option actor_rollout_ref.rollout.custom_async_server to allow users to customize the async server class.
2. Make load_extern_type more robust and support prefix like pkg:// or file://, while non-breaking to any existing features and supported paths.

Without this PR, it's impossible to use a customized version of AsyncvLLMServer in customized use case. We are currently using a set of ugly monkey patch to achieve this goal.
Ultimately I believe rollout.name and rollout.custom_async_server can be combined. But rollout.name is currently referenced in too many places. It's quite difficult for me to handle all of them.

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Test

I have tested on our internal pipelines. The new patch works as expected and the old async servers still work as usual.

API and Usage Example

Our config is something like this:

hydra:
  searchpath:
    - pkg://verl/trainer/config

defaults:
  - ppo_trainer
  - _self_

data:
  filter_overlong_prompts: false

actor_rollout_ref:
  rollout:
    mode: async
    custom_async_server:
      path: pkg://mypackage.verl.async_server
      name: CustomizedvLLMServer

High-Level Design

This PR is pretty straightforward.

Specific Changes

Update the docs. Update behavior in agent loop and async server manager. Update load_extern_type implementation.

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Important

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[wuxibin89]

Please move custom_async_server under agent field, async server is only used by agent.

[ultmaster]

moved