Add support for tls certificate update

[binoue]

This pull request addresses #3198.

In this pull request, the following changes are made.

• Add goroutine to load x509 certificate and add tlsConfig.GetCertificate handler to apply the loaded x509 certificate.
• Add cli option to adjust the interval of updating tls certificates.

[gravitational-jenkins]

Can one of the admins verify this patch?

[webvictim]

ok to test

[klizhentas]

setting expectations folks, the proposed design is very risky, as there is no explicit state machine for rolling this back in this design. I would want to have FSM like approach using signals similar to what we have for binary reloads.

[binoue]

Thank you for reviewing!
The pull request is intending to keep updating the certificate with a certain interval.
And if the certificate is not valid, it is ignored.

Therefore, if the certificate is invalid because of some operation,
the teleport process keeps working.

Considering that, we are not sure what risk this pull request has.
If we misunderstand, please let us know.

[russjones]

@binoue Take a look at how CA rotation is handled and has the ability to roll back to a prior state if needed. It was introduced in #1899, this is required by Teleport.

If you are interested in making the above changes let's proceed, otherwise it's best to close this for now.

[binoue]

@russjones

Thank you for your replay.

If you are interested in making the above changes let's proceed, otherwise it's best to close this for now

Okey, I will try to fix this pull request as soon as possible

[binoue]

@russjones

I checked your links, code, and documents.
I think I need to implement something like tctl proxy rotate for this issue.
This seems to become a big PR and to take much time.

In the original issue, a maintainer recognizes this as an important issue.
Is there any plan to implement this feature at some point?
If so, I will close this pull request.

[russjones]

@binoue Let's close this PR for now then. We don't have plans to address this at the moment, but we'll keep it in mind when planning the next milestone.