[feat] provide some way to suppress unused patch warnings

[J-ZhengLi]

Problem

The [patch] section in .cargo/config.toml is very useful to override certain dependencies (even if it's no uploaded to crates.io) of multiple projects.
But the downside is, for projects that do not use those patched dependencies, cargo will repeatedly show warnings about those being unused, which seems annoying over time.

Example

In .cargo/config.toml

[patch.crates-io]
foo = { path = "/path/to/foo" }
bar = { path = "/path/to/bar" }

after running

cargo init hello_world
cargo run

warning: Patch `foo v0.1.0 (/path/to/foo)` was not used in the crate graph.
Patch `bar v0.1.0 (/path/to/bar)` was not used in the crate graph.
Check that the patched package version and available features are compatible
with the dependency requirements. If the patch has a different version from
what is locked in the Cargo.lock file, run `cargo update` to use the new
version. This may also occur with an optional dependency that is not enabled.

Proposed Solution

possible solutions (might not be the best)

1. Add a configuration key under [patch]:

   [patch.crates-io]
   ...
   allow-unused = true

2. Skip unused patch warning entirely if the [patch] configuration is coming from config.toml

Notes

No response

[weihanglo]

The problem of patch-in-confifg affects more than warnings, see

• Patches in ~/.cargo/config.toml add [[patch.unused]] to Cargo.lock #15130
• unused [patch] in $CARGO_HOME/config.toml failed loading sysroot metadata rust-analyzer#19667

As as #15130 has called out, this feature was expected to have these defects. We might need to reconsider it as a whole.

Speaking of suppressing warnings, this is seemingly a duplicate of #11782, which is tracked in #12235, so close in favor of those. Thank you.