Add support for generating storage upload policies and signatures

[theacodes]

Details

A policy (and its associated signature) allows a web application to create a form that allows user agents to directly upload to Google Cloud Storage. This is a pretty useful feature and it would be great if this client library could make that easier.

A rough idea:

storage = google.cloud.storage.Client()

policy = storage.policy(
 bucket='my-bucket',
 acl='bucket-owner-read',
 succes_action_redirection=url_for('upload_success'),
 content_type='image/jpeg',
 content_length_range=(0, 1024*10))

encoded = policy.encoded
signature = policy.signature

for field_name, field_value in policy:
 # HTML field names and values

[EverWinter23]

@theacodes Hey, how does generate_upload_policy method accommodate for CORS policy if the origin is restricted to a particular domain? I'm trying to upload a bunch of images to a prefix using this method. It's running into CORS errors if I put restrictions on the origin, it works fine if I set the origin to *, but is there a way to make it work with restricted origin?

The CORS policy looks exactly like this apart from the origin:

[
 {
 "responseHeader": ["Access-Control-Allow-Origin", "Content-Type", "Authorization"],

 "method": [
 "GET",
 "POST",
 "PUT",
 "DELETE",
 "OPTIONS"
 ],
 "origin": [
 "sampledomain.com"
 ],
 "exposedHeaders": [
 "ETag"
 ],
 "maxAgeSeconds": 3000
 }
]

Getting CORS errors while upload, am I missing something or does this method not allow restrictions on the origin at all?