docs: add fido2 jar customization

[moabu]

add to https://github.com/GluuFederation/docs-gluu-server-prod/blob/4.4/docs/source/authn-guide/fido2.md a section on customizing the fido jar. If the user wanted to load a custom built jar steps to do that is needed,

https://github.com/GluuFederation/flex/issues/263

CE:

Place jar that is originally is sourced https://maven.jans.io/maven/io/jans/jans-fido2-client/1.0.1-SNAPSHOT/ into /opt/jans/jetty/jans-auth/custom/libs and restart auth-server, fido2 authn will work

CN:

The mount can be different /usr/share/java/ as the file gets moved in the entrypoint.

This should be tested.