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?

Comienza con la nota del README de service-clients enlazada y reproduce el error de credenciales en el ejemplo de Nova Speech-to-Speech. Comprueba cómo obtiene actualmente las credenciales aws_sdk_bedrock_runtime y compara después el comportamiento esperado de descubrimiento estándar de CLI/boto3 para SageMaker y ECS. Se considera terminado cuando se descubren las credenciales de AWS configuradas y compatibles sin requerir AWS_ACCESS_KEY_ID ni AWS_SECRET_ACCESS_KEY.

Escrito por el modelo de indexación a partir del texto del issue.