add vctl install-lib

[schandrika]

To install an optional library for volttron, a user has to run
poetry add --directory $VOLTTRON_HOME <library>

If --directory is missed, the command might still successfully if the current directory has a pyproject.toml file. This would be misleading to the user as he/she might think it got installed correctly and is available to VOLTTRON. However, volttron can access that library only if $VOLTTRON_HOME/pyproject.toml has an entry for the library.,

Feature request:
add a vctl install-lib command as a wrapper for poetry add --directory $VOLTTRON_HOME