Initial implementation of TLS client authentication (#2731)

* Add support for client TLS authentication

Signed-off-by: Alexandre Stein <alexandre_stein@interlab-net.com>

* make and use client authentication struct

* force StrictSNIHost if TLSConnPolicies is not empty

* Implement leafs verification

* Fixes issue when using multiple verification

* applies the comments from maintainers

* Apply comment

* Refactor/cleanup initial TLS client auth implementation
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