OpenPaasContactsCollection: Make the "contacts:contact:add" exchange configurable

[HoussemNasri]

The exchange name is currently hardcoded in the code. If OpenPaas decides to use a different name in the future, we will need to modify the code and redeploy a new version with the updated exchange name, which is not ideal.

To address this issue, we need to add a configuration option in the openpass.properties file to allow users to configure the exchange name.

Refs #1261

[chibenwa]

Low priority