bugfix on multimodal bedrock model

[libetl]

The image message follows the Converse model, which is a small json following this schema

{"format": "png" | "jpeg" | "webp",
"source": {"bytes": "<base64>"} | {"s3Location": "s3://bucket/..."}
}

Also, botocore is already converting the image to base64 (from the bytes property).
If we send it already in base64 it will encode it a second time.

Tested locally.