AWSCredentialsProvider for s3-extentions

- fileSessionCredentials property is added
- AWSSessionCredentialsAdapter is adapter class between aws sdk AWSCredentialsProvider to jets3t Credentials
- FileSessionCredentialsProvider will refresh its session credentials from the file
- S3StorageDruidModule should return different AWSCredentialsProvider on the config settings
one thing that might be helpful to some comment about how configs in the files must be structured. You can also consider having the config be in json format and using the Druid objectmapper to read the file contents directly into a java object.