Cleanup Makefile targets

[waliid]

Description

This PR simplifies our Makefile targets by using dedicated scripts for each one.

Changes made

• Scripts have been split in two directories, public and private.
  • Scripts located in public directory are use by our Makefile.
  • Scripts located in private directory are use by our GitHub workflows.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).

[defagos]

Two general comments which might already appear assigned to some code changes:

• Parameter management is likely better with getopts. Several parameters have been introduced an I think having a way to reliably manage them would be beneficial.
• I like the idea of having extracted shell scripts for everything. I am still a bit wondering if we shouldn't make each script standalone. Some scripts have been introduced to factor out common logic (e.g. interactively getting secrets, installing pkgx, installing bundler) but shell scripts are difficult to read, understand and maintain when scattered over several files, each possibly defining its own set of variables used in other files.