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?

Inizia dalla nota collegata nel README di service-clients e riproduci l’errore relativo alle credenziali nell’esempio Nova Speech-to-Speech. Verifica in che modo aws_sdk_bedrock_runtime ottiene attualmente le credenziali, quindi confronta il comportamento standard di rilevamento previsto di CLI/boto3 per SageMaker ed ECS. Il lavoro è completato quando le credenziali AWS configurate e supportate vengono rilevate senza richiedere AWS_ACCESS_KEY_ID e AWS_SECRET_ACCESS_KEY.

Scritto dal modello di indicizzazione a partire dal testo della issue.