Update hello template to use SQLAlchemy core

[devhawk]

Other Changes:

• manually expand template expressions instead of using mako.
  • Mako uses the same ${} expression syntax as we do in dbos-config.
  • updated code still uses ${} but ignores expressions that don't have known variable names (mako throws in that case)
• add --template option to dbos init
• rename template $package folder to __package to make mypy happy