This commit adds the user app gen tool for customer application
development. This tool completely decouples app development
from stx build, which means the app developers no longer need
to fetch stx code/build tool nor to build app by stx build system.
1. One command to package chart, generate manifest, checksum and
2. Supports local dir, git repo and tarball as chart source.
3. The app manifest abstracts a few important fields from armada
schema for user to lower the learning curve of armada.
4. Static value overrides allowed in app manifest
Signed-off-by: Mingyuan Qi <firstname.lastname@example.org>