Add cassandra-cdc backfill command as a Pulsar Admin Extension

[aymkhalil]

This patch adds initial support for running backfill-cli as a pulsar admin extension. I tested the happy path locally using the LS2.10_3 tarball:

bin/pulsar-admin cassandra-cdc backfill --data-dir target/export --export-host 127.0.0.1:9042 --export-username cassandra --export-password cassandra --keyspace ks1 --table sample4

in the client.conf I added the following line:

customCommandFactories=cassandra-cdc

Implementation notes:

• Added few DSBulk workarounds to make it class loader aware, and to have it run with a log4j log implementation. For more regarding the long term solution, please check https://github.com/datastax/dsbulk/issues/465 & https://github.com/datastax/dsbulk/issues/466
• Added a nar file task to be used as a pulsar admin extention
• Added basic CustomCommandFactory to show the happy path. This will be extended shortly with
  • client.conf settings related to pulsar client authentication
  • The exhaustive list of options avaible on the backfill CLI
• The backfill cli supports java 8 and 11 for now, and will not work as a pulsar admin extension with Pulsar 2.11 for example.