Support passing workload identity name to `require_access_token`

**Is your feature request related to a problem? Please describe.**
I want to use the 3LO flow with Identity outside of Runtime. As of now, this SDK falls back to the local auth settings expecting the `".agentcore.json"` which is not set by us. Hence, the SDK falls back to creating an Identity Workload but the agent's execution roles correctly gets an exception denied.

**Describe the solution you'd like**
I would like to use the decorator and pass the workload identity name. I would expect the decorator to get the workload access token and continue as is.

**Describe alternatives you've considered**
- Placing the `".agentcore.json"`
- Setting the `BedrockAgentCoreContext`

**Additional context**
Agent runs on Amazon ECS

首先定位 `require_access_token` 的 Python 实现，以及装饰器对 `.agentcore.json` 和 `BedrockAgentCoreContext` 的处理方式。跟踪 ECS/3LO 用例中如何选择工作负载访问令牌。完成标准是装饰器接受工作负载身份名称，并使用其令牌，而不要求本地身份验证设置。

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