Support passing workload identity name to `require_access_token`

[juliangrueber]

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