Support AWS IAM auth for AWS Bedrock

[salmanap]

Specifically, add support for role-based authentication, where Plano authenticates the Bedrock via an AWS IAM Role. For more details start here: https://docs.aws.amazon.com/bedrock/latest/userguide/security-iam.html

Note, we don't want to package the RUST SDK with Plano. It would now allow us to control failover to other providers neatly, not follow a consistent retry mechanism and other scenarios where we want to manage the binary payload using Envoy WASM ourselves. We are okay to bring in select Rust crates as utilities to help us as needed. For example, today we use aws-smithy-eventstream = "0.60" to help manage the binary frames in which Bedrock sends back LLM response streams.

Out of scope: long-living sessions keys. Only support for IAM role-based authentication.

[samadpls]

Would like to work on this issue. Can you please assign it to me?

[salmanap]

@samadpls assigned!