Feature Request: AWS Web Identity Token File (EKS IRSA) Support for Bedrock

Problem:
OpenCode cannot authenticate with Amazon Bedrock from Kubernetes/EKS environments using IAM Roles for Service Accounts (IRSA).

Proposed Solution:
Add support for AWS Web Identity Token File authentication by detecting AWS_WEB_IDENTITY_TOKEN_FILE environment variable and enabling the AWS credential chain when no bearer token is present.

Use Case:
Enable OpenCode to run in containerized workloads (Kubernetes/EKS) with IRSA, which is the standard AWS authentication method for pods.

Environment Variables:

• AWS_WEB_IDENTITY_TOKEN_FILE - Path to token file (set by Kubernetes)
• AWS_ROLE_ARN - IAM role to assume
• AWS_REGION - AWS region