Add option to use session token in S3 target credentials

[jessedadams]

The S3 target initialization takes an access key ID and a secret access key. Here is the relevant mercure code:

def create_client(self, target: S3Target):
   return boto3.client(
       "s3",
       region_name=target.region,
       aws_access_key_id=target.access_key_id,
       aws_secret_access_key=target.secret_access_key,
   )

However, some temporary S3 credentials also require a session token. Here is an example of the initialization:

client = boto3.client(
    's3',
    aws_access_key_id=ACCESS_KEY,
    aws_secret_access_key=SECRET_KEY,
    aws_session_token=SESSION_TOKEN
)

It would be very helpful to be able to specify a session token in the mercure S3 target, otherwise temporary credentials are unable to be used in mercure.

Thanks!