Feat/cors headers

[St4NNi]

CORS handling

PR implementing all necessary parts for handling CORS requests with S3.

CORS configuration can be added in two ways:

1. Via the PutBucketCORS S3 request (preferred)
2. By adding a Label with a manual cors configuration:

Example:

{
  "key": "app.aruna-storage.org/cors",
  "value": [
    {
      "id": "cors",
      "allowed_headers": [
        "*"
      ],
      "allowed_methods": [
        "GET",
        "HEAD"
      ],
      "allowed_origins": [
        "https://www.example.com"
      ],
      "max_age_seconds": 86400
    }
  ]
}

Changes

• Added PutBucketCors
• Added DeleteBucketCors
• Added GetBucketCors
• Added blanket impl for GetBucketLocation (This will always return us-east1)
• Add CORS config as app.aruna-storage.org/cors label to project

Fixes

• Fix HEAD object public permissions
• Fix Bucket modification (PUT/POST) permission check

resolves #29