[feature] S3 support (#674)

* feat: vendor minio client

* feat: introduce storage package with s3 support

* feat: serve s3 files directly

this saves a lot of bandwith as the files are fetched from the object
store directly

* fix: use explicit local storage in tests

* feat: integrate s3 storage with the main server

* fix: add s3 config to cli tests

* docs: explicitly set values in example config

also adds license header to the storage package

* fix: use better http status code on s3 redirect

HTTP 302 Found is the best fit, as it signifies that the resource
requested was found but not under its presumed URL

307/TemporaryRedirect would mean that this resource is usually located
here, not in this case

303/SeeOther indicates that the redirection does not link to the
requested resource but to another page

* refactor: use context in storage driver interface

internal/api/model/content.go

@@ -18,7 +18,10 @@
 
 package model
 
-import "io"
+import (
+	"io"
+	"net/url"
+)
 
 // Content wraps everything needed to serve a blob of content (some kind of media) through the API.
 type Content struct {
@@ -28,6 +31,8 @@ type Content struct {
 	ContentLength int64
 	// Actual content
 	Content io.Reader
+	// Resource URL to forward to if the file can be fetched from the storage directly (e.g signed S3 URL)
+	URL *url.URL
 }
 
 // GetContentRequestForm describes a piece of content desired by the caller of the fileserver API.