[keyserver] add aws proxy type for self-hosted keyservers

Summary:
This adds another proxy type to account for the aws load balancer sending user traffic to keyserver nodes instead of the user directly sending requests.

Depends on D12711

Test Plan:
Configured my self-host setup to use aws proxy. Was able to connect a debug and prod native build to the keyserver nodes. Console logged requests to confirm its
contents and headers were as expected

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12724