Make header methods and config public

[quangIO]

The PR proposes to make the config and helper methods inside Headers public. The current SxgWorker requires user to gather all certificates before hand. However, it may not be the case when a system stores certificates elsewhere or want to construct SxgWorker dynamically.

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[antiphoton]

Thanks for you contribution.

1. Making Headers public LGTM, but please split the headers change and worker config change into two PRs.
2. To construct SxgWorker dynamically, please use SxgWorker::new(config, cert, issuer), because this constructor also internally does some parsing work. If you need to construct a Config with certificates in formats other than PEM, you are welcome to set the PR to add more constructors to Config.

[twifkak]

Thanks for the contribution! I'll let @antiphoton review in full. I just wanted to leave one small (optional) suggestion.

[twifkak]

I recommend limiting .gitignore to project-specific artifacts, and using ~/.config/git/ignore for tool-related artifacts across projects.

[quangIO]

Thanks for the code review. I will close the PR in favour of #198 and #199.