[Feature]: transition stuart commands to subcommands

[Javagedes]

the current layout of stuart commands (stuart_update, stuart_build) is relatively uncommon nomenclature for groups of common commands. The typical design pattern is to have a command with multiple subcommands (think git with git checkout, git branch, etc.

edk2-pytool-extensions should follow this pattern by setting up a single entry point, stuart, that will invoke the different invocables depending on the command provided. This will be handled via argparser's command and subcommand options.

stuart_init -> stuart init
stuart_update -> stuart update
stuart_pr_eval -> stuart pr_eval
stuart_build -> stuart build

Relies on: #556 , #557