Add support for 'additionalLibrary' in .gitmodules

We introduced new 'Additional Library' module type. It meas Module with type 'Additional Library' won't be part of official Qt6 release but delivered separately as source package and with conan build support.

With Qt6.0 we had 6 additional libraries and with 6.1 there will me ~ 10 more. So we need to automate

• Dependency updates for additional libraries as well; otherwise it will be hard to keep those updated
• Version number bumps for additional libraries as well

To be able to do that we have agreed to add new 'status = additionalLibrary' in .gitmodules.

And modules having 'status = additionalLibrary' in .gitmodules

• CI should handle those like 'ignore' ones; That way we can get essential+addon package integration done even there is some issue with additional libraries; those can be fixed later on
• Dependency update bot should update additional libraries dependencies.yamls in separate round so that problems in an additional library won't disturb dependency update round of essential and addon modules.
• branching/version number bump script should handle 'additionalLibrary' similarly that addon/essential: Do the branching & bump if version number matches