feat(bedrock): add support for AWS_BEARER_TOKEN_BEDROCK

[Ashutosh0x]

Fixes #1079.

This PR adds support for Bearer Token authentication in the AnthropicBedrock client.

Changes

• Added aws_bearer_token parameter to AnthropicBedrock and AsyncAnthropicBedrock.
• Automatically checks for the AWS_BEARER_TOKEN_BEDROCK environment variable.
• Uses Authorization: Bearer header if a bearer token is provided, skipping SigV4 authentication.

Verification

• Verified with verify_bedrock_auth.py by checking request preparation and environment variable handling.

[karpetrosyan]

I saw many duplicate PRs opened by you.
We appreciate your contributions, but if we already have a PR resolving an issue, we don’t need a second one—especially when it’s essentially the same as the first.
Closing this as it's the duplicate of #1080