S3 support in Testgrid

[rahulbharuka]

Currently, Testgrid works only with GCS.
https://github.com/GoogleCloudPlatform/testgrid/blob/master/util/gcs/gcs.go#L94

What would you like to be added:
It would be very useful to be able to use AWS/Azure blob services in addition to GCS.

Why is this needed:
Some companies are using non-GCS blob storage extensively. It would be convenient if they can extend its use to testgrid.

[rahulbharuka]

/help-wanted

[rahulbharuka]

/help

[fejta]

We have more than enough work supporting GCP users right now. I suspect it will be awhile before AWS support becomes something we can afford to maintain. And S3 data is not something the web app can render.