Feature Request: Add mutable abs_repo_base to packs actions

It would be nice to select an alternative install location for st2 pack install.

I added another directory to st2.conf > [content] > packs_base_paths that I would like to use for a private set of packs. https://github.com/StackStorm/st2/blob/55e3b02b9b06720129e14316c21cff0cf7619b87/conf/st2.conf.sample#L90-L91

These private packs have ansible playbooks in them, but there's no easy way to determine whether they are in a packs.dev folder during development (like on docker) or somewhere else (like the folder that I want to put them in). So, I have a datastore key where I list the folder that contains these packs. I would prefer to keep these packs, and the playbooks that are tightly coupled to specific StackStorm actions, outside of /opt/stackstorm.

The download action has an abs_repo_base parameter: https://github.com/StackStorm/st2/blob/55e3b02b9b06720129e14316c21cff0cf7619b87/contrib/packs/actions/download.yaml#L13-L16

Could that be marked as not immutable? And can we add that parameter to packs.install?

Start with conf/st2.conf.sample around [content] > packs_base_paths and contrib/packs/actions/download.yaml, then locate the implementation of st2 pack install and packs.install. Compare how abs_repo_base is handled for download, and verify that installation can use an alternate pack location without changing the existing default behavior.