🤖 feat: Add support for Amazon Bedrock

Add Amazon Bedrock as a supported AI provider using @ai-sdk/amazon-bedrock
with flexible AWS authentication via @aws-sdk/credential-providers.

## Authentication Methods (in priority order)

1. **Explicit credentials in providers.jsonc** (accessKeyId + secretAccessKey)
2. **AWS_BEARER_TOKEN_BEDROCK** - Bedrock API key (simplest auth)
3. **AWS credential provider chain**:
   - Environment variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY)
   - Shared credentials file (~/.aws/credentials)
   - EC2 instance profiles
   - ECS task roles
   - EKS service accounts (IRSA)
   - SSO credentials
   - And more...

## Configuration

Region is always required (config or AWS_REGION env var).

Simplest setup with Bedrock API key:
```bash
export AWS_REGION=us-east-1
export AWS_BEARER_TOKEN_BEDROCK=your-api-key
```

Or explicit credentials in providers.jsonc:
```json
{
  "bedrock": {
    "region": "us-east-1",
    "accessKeyId": "...",
    "secretAccessKey": "..."
  }
}
```

## Supported Models

Model string format: bedrock:<model-id>
Example: bedrock:anthropic.claude-3-5-sonnet-20241022-v2:0

Closes #764

Author: ibetitsmike