Create event handler method for S3 keypair

[alanking]

Currently, irods_sync script has an --s3_keypair option which must point to a file with an S3 keypair. We should turn this into an event handler method so that the options for getting S3 keypairs for S3 scans can be expanded. Maybe something like this...

@staticmethod
def s3_keypair(meta):
    return "<S3_ACCESS_KEY>", "<S3_SECRET_KEY>"

This will allow the client to implement whatever means necessary to get S3 credentials. However the access key and secret key are acquired is an implementation decision, ultimately returning 2 strings to use with the S3 client in the ingest tool.

The meta argument could be useful to give some context around the job being performed.

[trel]

Love it.