Full boto-like credential configuration support

Trying out a Nova Speech-to-Speech example today, I was surprised `aws_sdk_bedrock_runtime` didn't correctly pick up my configured AWS credentials - with the below error:

```
aws_sdk_bedrock_runtime.models.ServiceError: AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY are required
```

[This note in the readme](https://github.com/awslabs/aws-sdk-python/blob/0090493b47493aea886c10ee9626755b36481f12/dev-guide/service-clients.md?plain=1#L21) seems to suggest there's a known limitation here - but is there an expected timeline for this library to align to full standard CLI/boto3 credential discovery?

It was straightforward enough for me to work around on my corp laptop, but I'm worried that there are AWS environments e.g. SageMaker and [ECS](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-environment-variables.html) where AWS credentials are available and people might want to run this connector, but AFAIK these specific environment variables are not set?

从链接的 service-clients README 说明开始，在 Nova Speech-to-Speech 示例中复现凭证错误。检查 aws_sdk_bedrock_runtime 当前如何获取凭证，然后将其与 SageMaker 和 ECS 预期的标准 CLI/boto3 发现行为进行比较。当无需 AWS_ACCESS_KEY_ID 和 AWS_SECRET_ACCESS_KEY 即可发现受支持的已配置 AWS 凭证时，即视为完成。

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