Add support for EFS POSIX profile

[RichardHeywood]

When using a Transfer Family server backed by EFS, it's required to provide a PosixProfile in the lambda response. This PR adds the ability to define that profile within your AWS Secret.

[RichardHeywood]

This requires a secret using the following example format for the POSIX profile:

{"Password":"myPassword","Role":"arn:aws:iam::<account-id>:role/<role_name>","UserId":"<user_id>","HomeDirectory":"/<efs_filesystem_id>","PosixProfile":"{\"Uid\": 33, \"Gid\": 33}"}

The above example uses 33:33 for php-fpm, but that should be updated based on the Transfer Family user.