Add non-lint `advice` messages controlled through a `[advice]` lint table

[epage]

Problem

We have many situations where people would find additional information useful for new users at the cost of output blot.

Potential cases

• cargo news see more Cargo.toml keys and their definitions
• cargo add calling out the lib.name (#15887)
• misnamed config (#8631)
• suggest [workspace] when using path dependencies (#7589)
• unused config relative to a --manifest-path (#2930)
• No locking available (#16184)

Proposed Solution

Add a non-lint advice. This is meant to be slightly more verbose output of helpful suggestions that the user can turn off for less output, similar to git.

A fundamental difference between a lint and advice is that a lint is focused on the project while advice is focused on the user.

Notes

#9936 would be helpful so we can easily tell people how to disable advice

[epage]

Created this atm to help collect use cases s we can better evaluate this

[hanna-kruppe]

Would this table live in workspace/package manifests or in configuration? The use case feels more like a per-user config thing, but the lints table is workspace/package level and I didn’t see any explicit mention of config vs manifest.

[epage]

Config. I guess in my rewording I didn't say it explicitly but

• it is labeled as configuration, rather than manifest
• I call out this is user focused, rather than project.