[feature] Merge "Client certificate support"

[rasmuslos]

Hey,

I stumbled upon this project while looking into securing my network traffic with mTLS. There is an open PR (NginxProxyManager#2956) for TLS client certificate configuration inside NPM, but as far as I can tell, it has not been merged, even though the feature is completed, for about two years now.

While reading the README, I noticed that an upstream PR got merged, so I was wondering if it would be possible to do the same for mTLS.

Thanks,
Rasmus

[Zoey2936]

maybe in the future, the PR was not rebased in two years (thats not a big problem), but currently I want to work on different things

[Zoey2936]

I just saw that this a duplicate of #842