set a custom auth header when requesting docs.rs origin servers

[syphar]

This is coming from this discussion on zulip

custom_header is not documented, but it exists: terraform-aws-modules/terraform-aws-cloudfront#69, linting seems to accept it too.

The value for the secret has to come from another source.

On other thing that is also missing: adding the secret value to the new docs.rs setup which is managed by simpleinfra here.

[syphar]

@Mark-Simulacrum here is the change itself, only the secret source would be missing.

Since the docs.rs server itself is still manually managed, adding the secret check to our config would be too