feat: add "cargo" to deb build dependencies for charm plugin

[james-garner-canonical]

This PR adds "cargo" to the charm plugin build dependencies for deb based charms.

I don't know if a similar fix is needed for yum and dnf systems -- review appreciated here.

This will prevent various charm-libs from needing to document that including them requires adding cargo to build-packages in charmcraft.yaml, and prevent charmers from having to do this, since cargo will be included be default, and python packages requiring a rust compiler to build (e.g. pydantic) will work out of the box.

See issue:

• charm plugin should include rust compiler in build dependencies #2004

[lengau]

This implementation may break a charm if the user installs their own cargo (and rustc) through another mechanism such as rustup, which might be required if the base has too old a version of rustc.

Please look at the way the poetry plugin and its environment validator implement the checks