Add support for uncompressed .log files in s3 generated by classic load balancers

Currently the s3 module assumes all log files are gzipped (https://github.com/grafana/loki/blob/46c6118eb2a2d56406042431dfbed288df6e924b/tools/lambda-promtail/lambda-promtail/s3.go#L152)

Classic load balancers seem to be the only class of AWS LB that sends it's logs to S3 in an uncompressed format

It would be ideal if we could check to see if the file is gzipped or not before trying to read it as such as it currently results in the following error.

The module already allows the .log file extension.

This is the docs source for log file format details https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-access-logs.html#access-log-file-format