Added TLS/SSL support

[jonasgeiler]

Closes #243

Description

This PR adds support for passing TLS/SSL certificates/keys to the API server, which makes it possible to use encrypted HTTPS connections for communicating with Gotenberg.
I basically added two new flags, --api-tls-cert-file and --api-tls-key-file, which can be set to the location of the TLS/SSL files. Then, when starting the API server, it will use the StartTLS method instead of the StartH2CServer method to start a server with HTTPS support and everything should be the same from there on out.

TODO

• Implement
• Add tests
• Run a Gotenberg instance manually and check everything is working

Open for review!

[gulien]

A few comments otherwise 👍 thanks a lot!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.91%. Comparing base (e50de42) to head (e5e8434).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #888      +/-   ##
==========================================
+ Coverage   91.89%   91.91%   +0.02%     
==========================================
  Files          46       46              
  Lines        4009     4021      +12     
==========================================
+ Hits         3684     3696      +12     
  Misses        230      230              
  Partials       95       95              

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[jonasgeiler]

@gulien should I try to get better test coverage by making a scenario where it throws an error in api.Start?

[jonasgeiler]

@gulien should I try to get better test coverage by making a scenario where it throws an error in api.Start?

Okay I'm not sure how to do that 😅 Tried passing in invalid TLS files but the error is thrown with Fatal and not returned from api.Start, so I can't catch it in any way I think...
Any suggestions? Otherwise I'll just leave it away.

[gulien]

@jonasgeiler My bad; actually, I just want most of the lines you've added to be covered. But on scenarios like this one it's simply not feasible 😄

[gulien]

Thanks a lot for your time @jonasgeiler!

The release gotenberg/gotenberg:edge should be available in ~1 hour. Even if marked as "edge", it's pretty stable so don't wait for the "official" 8.6.0 variant that will come later this week.

[jonasgeiler]

@gulien Awesome! Thanks! 🎉