NATS TLS specify CA and client TLS authentication (#8389)

- added ability to specify CA for self-signed certificates
- added option to authenticate using client certificates
- added unit tests for nats connections

pkg/event/target/testdata/certs/nats_client_key.pem

@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIIEogIBAAKCAQEA2wuBLsLQEOx+FZBXK4UaumX0hYbIBC9IQLN6l2/FxTMInlBN
+KO0yr7ZduSjOyYnRYq1dj5za9i40xy2F83MuafwozEOgKozAtc0w7yUu4Xn3ikfQ
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+Geaq85ItKcb3KcX5aUTtnKQfD5WwitGyX/k3CQIDAQABAoIBACrO7Mg+unsUPO/p
+7Z9LvBWBp3ARDzYCJ5S9fs/pwDTx7FVETFAbSzSb52UwXHl2vb0TNJ3EgeZq0VW7
+x9n0QLXl2fNRpBOsvlzJZS7XjXnzZDVaI4+dF8c4YzCl8LtY3QxhVm7VLgIdf3Uc
+Tc2fgOiePwGNjOetwfMTxtsYqqJK4Ang/nop6WIZtJxlq/H4D/OueDScv8yM96WP
+Lb+5YMrlTtA79TTn44hdd/IVJBV74UK335bWaV2gqop/r/kymYcmIS6eQ8Fx8Au8
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+JzSCYiECgYEA/0e1h7XVHVjRI/hzlpZlHomw4SYXc1R5DBljsD+0baNWdgwShdiA
+qZyhVl1/WJEK6r+tT/a0rGcIz1CkjPvjOf66+FVwukqp4UDdeZA28CC6GvbAlBW7
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+ami6EkbNwnA5ZFBN5FCQ8zVCnga8Toa1vrAhJXWKpU1LAdU5DYxUjFt626HqKrYm
+Fg3SOXyAyc3Tb8xI5Fh2zE8RxC+r3qwxoVjPWM/8eYNwHHMUBGCorIh9RfIU3seT
+qATSCHwnKv9lNXzKoHNaLQKBgGURdkkn1mrFmCTnXYP06Sm57R1U18Opc0WEDqar
+JZyw4TF8eKqnUr9GG12UU7ob06i10+bqEIbyB9G17UxafJxhwf8nh2xD0tzJ0m1d
+AfFgGB6z558n2T0Nu+EGkQvN9Ye9kgUs8apMArOuEq6X/p/YWUmj3wZbIxjgbGxf
+W82lAoGATTwJCXE6y+D5Vc5I9BTPgJd6Wg/6n+BAmadt3lG/VbPxndQBudCNii0Z
+C7uyMrbK0RVY+DZAFuZWLskIoU+t82HgIsQNAH5KPz27PCWq+DznrARp+maKXj5f
+f5Vsvz6KBzM8Lnts3m20AYcRP4meBa6HNHLnYGedTbRLKYobOgg=
+-----END RSA PRIVATE KEY-----