Make TLS-Certificate configurable

Currently the proxy generates certificates on every start for dev purposes. This commit adds an option to make this behaviour configurable.
This also removes the dependency on konnectd`s crypto code and copies it instead, as this library is a first version which is not meant for usage by other services.

A proper cert-generation lib should be added to ocis-pkg instead. Then this code can be refactored to use it.
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