New function SSLServer::update_certs. Allows to update certificates while server is running

[RainerSchielke]

We want to update the ssl certificates without stopping the httplib server. The new function does this in a safe way, because it uses the ctx_mutex_

[yhirose]

@RainerSchielke thank you for the pull request. Could you please add a unit test for this change?

[RainerSchielke]

@RainerSchielke thank you for the pull request. Could you please add a unit test for this change?

Ok I willt try it, but I cannot find the certificates used in the test eg. "./cert.pem"

[yhirose]

@RainerSchielke sorry for the late reply. Thanks for your contribution!