This commit reworks TLS configuration of both servers and endpoints in
order to provide a more flexible API. We now add options to configure
the selected TLS library using the appropriate 'native' configuration
structures, as well as retaining the existing simplier interface which
is compatible with both.
The new API can also be easily extended to support simple interfaces for
configuring mTLS and a range of other options without creating sprawl
in the builders for `Server` and `Endpoint`.
