feat(kad): add mode getter on Behaviour (#5573)

## Description

Small PR adding a getter for the `mode` attribute of the
`kad::Behaviour` in order to get the mode that the DHT is operating in,
at the moment.

In our project, we needed to expose an API endpoint which included the
mode that the DHT was operating. Having a getter was necessary so we are
upstreaming this change.

## Notes & open questions

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## Change checklist

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- [x] I have performed a self-review of my own code
- [x] I have made corresponding changes to the documentation
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] A changelog entry has been made in the appropriate crates

protocols/kad/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 - Expose a kad query facility allowing specify num_results dynamicly.
   See [PR 5555](https://github.com/libp2p/rust-libp2p/pull/5555).
+- Add `mode` getter on `Behaviour`.
+  See [PR 5573](https://github.com/libp2p/rust-libp2p/pull/5573).
+
 
 ## 0.46.2
 

protocols/kad/src/behaviour.rs

@@ -1111,6 +1111,11 @@ where
         }
     }
 
+    /// Get the [`Mode`] in which the DHT is currently operating.
+    pub fn mode(&self) -> Mode {
+        self.mode
+    }
+
     fn reconfigure_mode(&mut self) {
         if self.connections.is_empty() {
             return;