feat(bedrock): add support for AWS_BEARER_TOKEN_BEDROCK env var

[Ashutosh0x]

feat(bedrock): add support for AWS_BEARER_TOKEN_BEDROCK env var

Summary

This PR adds support for the AWS_BEARER_TOKEN_BEDROCK environment variable in the Bedrock client, enabling bearer token authentication as an alternative to SigV4.

Problem

Currently, the Bedrock client only supports SigV4 authentication. Some users need to use bearer token authentication for specific deployment scenarios.

Solution

• Added get_bearer_token() function to _auth.py to retrieve the token from the environment
• Updated _prepare_request in both sync and async Bedrock clients to prioritize bearer token if AWS_BEARER_TOKEN_BEDROCK is set, falling back to SigV4 otherwise

Usage

import os
os.environ["AWS_BEARER_TOKEN_BEDROCK"] = "your-bearer-token"

from anthropic import AnthropicBedrock

client = AnthropicBedrock()
# Client now uses bearer token auth instead of SigV4

Fixes #1079