refactor: ensure X-Exodus-Version works for synthesized responses too

[dichn]

If the logic for setting X-Exodus-Version lives only in origin-response,
it can't work for responses generated directly without going to origin,
e.g. /listing files, /_/cookie, 404 errors and maybe some other things
in the future.

Implement it also in origin-request, in a way which ensures it can't be
forgotten as handler is extended.

[dichn]

I tested the listing request to my aws cloudfront, looks good. (push this change to master branch temporarily for checking the result)

$ curl -I -H "x-exodus-query: true" -H 'Cache-Control: no-cache' https://<cloudfront-url>/content/dist/rhel/server/5/5.7/listing
HTTP/2 200 
content-type: text/plain
content-length: 17
server: CloudFront
date: Mon, 18 Apr 2022 04:07:22 GMT
cache-control: max-age=600
x-exodus-version: 2022-04-18T03:38:36+0000 6b94a45b42cf5c474acf8db848ab33c8301a3a1a
x-cache: Miss from cloudfront
via: 1.1 ec75fb3eaaf9729edc3317dd95f42eea.cloudfront.net (CloudFront)
x-amz-cf-pop: NRT57-P1
x-amz-cf-id: AePlQBe_C3Oe7g0k7v0xJx7HFOuVANlIEddyKXiR0d0xg7FpHSDj_Q==