Add support for HTTP headers on proxy requests

**Is your feature request related to a problem? Please describe.**
I would like to have relatively "private" connectivity from my Amplify site to my VPC. I am planning on using the ALB OIDC authentication feature, but it would make security happy to ensure only traffic hitting my API is coming from the static site deployed on Amplify Console. This could be used along with #36 for hosting internal apps.

**Describe the solution you'd like**
I would like to be able to include a HTTP header on proxied requests. Then I can use AWS WAF attached to my ALB to validate that it is present.

The ability to use PrivateLink to send traffic to my VPC would be really nice, but I know that's generally more region-based and would be a CloudFront feature request.

No repository files, tests, or entry points are identified in the issue. Start by locating the proxy-request path and reviewing how AWS Amplify forwards requests to an ALB; define done as allowing a configured HTTP header to reach the proxied request for AWS WAF validation.

Written by the indexing model from the issue text.