Add a unit test for external key provider

Add a unit test for external key provider

Tests:
- Check SIGNATURE and KEYMGMT methods can be fetched
  from the provider
- Load sample RSA and EC keys as management-external-key
  and check that their sign callbacks are correctly exercised:
  with and without digest support mocked in the client
  capability flag.
 -Test generic key load and signature

v4: 16/18 and 17/18 of v3 squashed into one patch

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20220120161616.13447-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23608.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>