[breaking change detector] easy to apply the tool to generate changelog for all packages

When I try to apply the tool to generate changelog for SDKs with command `PS D:\dev\azure-sdk-for-python\sdk\compute\azure-mgmt-compute> tox run -c ../../../eng/tox/tox.ini --root . -e breaking -- --changelog`, I have to add the package name in [RUN_BREAKING_CHANGES_PACKAGES](https://github.com/Azure/azure-sdk-for-python/blob/4e048f89a6e244a530d9d267883403197f7ab78a/scripts/breaking_changes_checker/breaking_changes_allowlist.py#L10C1-L10C31). It is very inconvenient.

I understand `RUN_BREAKING_CHANGES_PACKAGES` is a switch to check breakings for SDK in CI. But now it is also used when I try to generate changelog. I hope we could split changelog and breakings into 2 independent function.