Modify Submodule Update bot to allow syncing a specific repo's branch with the main round

There is a need for certain projects to maintain a different branching structure than the rest of Qt, despite depending on a specific branch of qt for dependencies.

Submodule Update bot should parse repos added by parameter to allow passing a specific branch for a given repo. Suggested to use :: as the delimiter

Example
> python dependency_update.py --branch 6.8 --repos tqtc-boot2qt/qtsaferenderer::2.2 tqtc-boot2qt/qtsaferenderer-runtime::2.2