Use mbedtls_ssl_export_keying_material()

Use mbedtls_ssl_export_keying_material()

Mbed TLS now has an implementation of the TLS-Exporter feature (though
not yet in a released version). Use it if it's available.

v2: Rebased, changed feature detection in configure.ac

Change-Id: I1204bc2ff85952160a86f0b9d1caae90e5065bc4
Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20250603140414.10970-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31856.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>