feat: Support AWS_BEARER_TOKEN_BEDROCK

- [x] I have looked for existing issues (including closed) about this

## Feature Request
Support AWS_BEARER_TOKEN_BEDROCK for Bedrock inference calls.

### Motivation
Bearer tokens make prototype iteration slightly easier when working with Bedrock.

### Proposal
Haven't looked deeply; this might "just work" if this crate uses the aws-sdk-bedrock crate.

### Alternatives

The standard AWS_SECRET_ACCESS_KEY works for production workloads.

Check if the aws-sdk-bedrock crate already supports AWS_BEARER_TOKEN_BEDROCK. Look at the authentication flow in the codebase, likely in a module handling AWS SDK configuration. Verify how inference calls are made and where credentials are injected. Test with a bearer token to see if it works or what changes are needed.

索引モデルが issue の本文から書いたものです。