Research proxy updates and options

[apburnes]

Based on the updates to cloud-gov/pages-core#4101, research and outline different possibilities for the sites proxy.

Background

An improved proxy could support options like:

• Unlimited domains per site
• Dedicated domain to host large file storage
• Unique preview domain per branch that hosts files from the root ("Removes need for previews to use pathing /preview/org/repo/branch/...)
• Add basic auth to site previews

Acceptance Criteria

• Document different possibilites
• Document architecture of each possibility
• Document changes to deployment and management

Security Considerations

• How can the proxy improve security and flexibility