Add a lint to automatically check for unused dependencies

[alamb]

          Perhaps we could enable a lint to test for this?

Originally posted by @tustvold in #6792 (review)

[findepi]

for context, in the past i tried using cargo machete, but was disappointed by false positives (maybe because i run it on a complex project). For #6792 i used RustRover's builtin capabilities. But even then I had to solve false positives manually. In apache/datafusion#13541 there was also one false positive (apparently due to an imperfectly implemented macro / limitation in RR -- depending on how we look at it)

I wish I knew how to make this automated.

[tustvold]

unused_crate_dependencies is a lint that can be enabled at the workspace level