feat(lambda-at-edge): support custom headers (with caveats)

[dphang]

This continues: #587

• Supports custom headers on pages and API handlers

Caveats: because of the way the default handler is currently architected (using S3 origin + origin response), any path that hits the S3 origin has its request.uri rewritten to the S3 path. This includes SSG pages, SSG data requests, public files, etc. Because of this, to have a custom header on these paths, you will need to specify the S3 key rather than the path. This is different from Next.js.

For example: if you have an SSG page at path /my-page, you will need to specify the S3 key as the header source in next.config.js, which will probably be something like /my-page.html.

Also, you can't add headers to next/static/* or static/* paths, as those do not have origin handlers attached to them.

In the future we may improve/fix this to be more in line with Next.js (e.g by possibly getting rid of the S3 origin and having the origin request handler make requests to S3 directly).

Tests

Added unit and e2e tests.