Specify depth of dependency traversal in bans.allow

[ethanuppal]

Is your feature request related to a problem? Please describe.
See discussion in #750

Describe the solution you'd like
An allow-depth field that (1) when unspecified defaults to infinity, and (2) when specified to a positive integer limits the scope of the allow field to those in the first allow-depth breadth-first-search layers of the dependency graph starting from the root dependencies

Describe alternatives you've considered

• Simply put every dependency in the allow
• Use comments in Cargo.toml

Additional context
#750