Add support for `baseBranch` flag

[umpox]

• Adds support for baseBranch flag in the CLI
• Changes default baseBranch to main

What do you think @siddharthkp?

Could still support master, but we might need to update the summary to text to use a non-specific word as we can't be sure if we're falling back to main or master unless we check. e.g. larger than master could become larger than baseline