Add ability to renv::restore to restore only a subset of packages from the library

[vspinu]

Would be nice to be able to restore only a subset of the library. Boils down to the new packages argument to renv::restore. But I am not sure how well this would play with renv::status.

The primary use case is to use with docker in production which commonly requires only a small subset of all packages used.

It might be useful in interactive contexts as well, such as when for collaborators wants to quickly use/test only part of the functionality of the project.

This one is narrower than #253 but would quickly address a major use case.

[vspinu]

Wow, this was quick :) Thanks a bunch for this!

[kevinushey]

Happy to help! Glad it wasn't too difficult to implement.