Add AWS Bedrock authentication support for Anthropic

[bheemreddy-samsara]

Summary

Add support for AWS Bedrock as an alternative authentication method for Anthropic Claude models. This enables users who have Claude access through AWS Bedrock to use Ash without needing a separate Anthropic API key.

Motivation

• Users with AWS Bedrock access can leverage their existing AWS credentials
• Organizations that provision Claude through AWS can use Ash without separate API key management
• Aligns with how other tools (like Claude Code) support Bedrock authentication

Proposed Configuration

[anthropic]
auth_method = "bedrock"  # "api_key" (default) or "bedrock"
bedrock_region = "us-west-2"  # Required when using Bedrock

[models.default]
provider = "anthropic"
model = "claude-sonnet-4-20250514"  # Standard model names work

Implementation Details

1. New optional dependency: anthropic[bedrock] for Bedrock client support
2. Config changes: Add auth_method and bedrock_region to ProviderConfig
3. Model translation: Map standard Anthropic model names to Bedrock inference profile IDs (e.g., claude-sonnet-4-20250514 → us.anthropic.claude-sonnet-4-20250514-v1:0)
4. Authentication flow: When auth_method = "bedrock", use AsyncAnthropicBedrock client with AWS credentials from environment

Installation

pip install 'ash[bedrock]'

Users would configure AWS credentials via standard methods (env vars, ~/.aws/credentials, IAM roles).

[bheemreddy-samsara]

Happy to create a PR for this - I have a working implementation ready.