add option for extra HTTP headers containing checksums

[yol]

adds an rfc1864 compatible content-md5 header and analog extensions for sha1 and sha256 if the option CheckSumHeaders is true the following extra http headers will be added:
Content-Md5: I9rjdRkQr90tbzCzZtDy4A==
Content-Sha1: kvbBunf6Il0/5oMcuItvYcaHN9c=
Content-Sha256: ktVLJsGP3LWFWjCsQLAXtIWE1kd3b+bXyNWmoFbxvV0=

[elboulangero]

What is the use-case?

[yol]

It's used by kodi to verify that the mirror that it gets redirected to is delivering the expected file unaltered

[elboulangero]

For reference, these are the mirrorbits changes on kodi's side, from 2018: https://github.com/xbmc/mirrorbits/pulls?q=is%3Apr+is%3Aclosed

I did some research and RFC-1864 seems to be considered deprecated since HTTP/1.1 (grep for Content-Md5 in RFC-7231). Plus, Content-Sha1 and Content-Sha256 are not part of this RFC, so they would be custom additions.

OTOH, there's this old discussion: #49 (comment), where etix discussed RFC-6249 and adding support for a Digest field.

It seems to me that RFC-6249 would be a better way to go.

RFC names:

• RFC-1864: The Content-MD5 Header Field
• RFC-6249: Metalink/HTTP: Mirrors and Hashes
• RFC-7231: Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content

[yol]

We do not have the option to use any different structure/headers,, since Kodi expects the Content-* headers and we cannot retro-patch every active installation out there. So at least for us, this would not be an option. We can also keep it alive in our fork if that is preferred.