add custom_header variable to send custom headers to origin

[bkero]

what

• Adds the ability to specify custom headers that are sent to the origin for requests

why

• To facilitate the common pattern of only allowing traffic to origins that come form a certain CloudFront CDN, or for other use cases

references

closes #46

[osterman]

/test all

[mihaiplesa]

Seems to have failed bats with:

ERROR: unsatisfiable constraints:
  json2hcl (missing):
    required by: world[json2hcl]
make: *** [Makefile.alpine:4: deps] Error 1
Error: Process completed with exit code 2.

[mihaiplesa]

@osterman any chance we can retest this?

[joe-niland]

/test bats

[joe-niland]

@mihaiplesa thanks for your contribution!

[mihaiplesa]

Credit goes to @bkero here at Brave. Thanks for helping on a Sunday!

[joe-niland]

@mihaiplesa my mistake!
Thanks for the PR @bkero !