Add API key auth way for Anthropic Bedrock in Agent Panel

[lemorage]

Now we must need the full AWS credentials to use AWS Bedrock for Anthropic models in Zed Agent Panel. There is a simpler way, that we can just use API keys for auth. It would be super if we can support that.

https://docs.anthropic.com/en/docs/claude-code/amazon-bedrock#2-configure-aws-credentials
https://aws.amazon.com/blogs/machine-learning/accelerate-ai-development-with-amazon-bedrock-api-keys/