Add internal lint `derive_deserialize_allowing_unknown` (#14360)

Adds an internal lint to check for `#[derive(serde::Deserialize)]`
without
[`#[serde(deny_unknown_fields)]`](https://serde.rs/container-attrs.html#deny_unknown_fields).

Today, if you run Clippy with the following clippy.toml, Clippy will
produce a warning, but there will be no accompanying note:
```toml
# In the following configuration, "recommendation" should be "reason" or "replacement".
disallowed-macros = [
    { path = "std::panic", recommendation = "return a `std::result::Result::Error` instead" },
]
```
```sh
$ cargo clippy
    Checking a v0.1.0 (/home/smoelius/tmp/a)
warning: use of a disallowed macro `std::panic`
 --> src/lib.rs:2:5
  |
2 |     panic!();
  |     ^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_macros
  = note: `#[warn(clippy::disallowed_macros)]` on by default
```
The underlying problem is: the enum that derives `serde::Deserialize`
([`DisallowedPathEnum`](https://github.com/rust-lang/rust-clippy/blob/81643e297cf44ce3c7648b8443fc4d6592fa81eb/clippy_config/src/types.rs#L47))
does not have the attribute `#[serde(deny_unknown_fields)]`.

This lint identifies such problems by checking trait `impl`s. An
alternative I considered was to walk `clippy_config::conf::Conf`
directly. However, that would not catch the `DisallowedPathEnum` case
because it [is not used in `Conf`
directly](https://github.com/rust-lang/rust-clippy/blob/81643e297cf44ce3c7648b8443fc4d6592fa81eb/clippy_config/src/types.rs#L31).

Just to be clear, no one asked for this. So I hope the maintainers do
not mind.

changelog: none

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