Add fn to find out if Host can be finished (#1182)

### What
Add fn to find out if Host can be finished.

### Why
So that the SDK can ask the Host if it has no more references left and
it is able to be finished without calling try_finish which is
destructive.

I plan to use this in:
- stellar/rs-soroban-sdk#1137

soroban-env-host/src/host.rs

@@ -486,9 +486,20 @@ impl Host {
         self.as_budget().with_shadow_mode(f, e)
     }
 
+    /// Returns whether the Host can be finished by calling
+    /// [`Host::try_finish`].
+    ///
+    /// Returns true if the host reference is unique, refcount = 1.
+    pub fn can_finish(&self) -> bool {
+        Rc::strong_count(&self.0) == 1
+    }
+
     /// Accept a _unique_ (refcount = 1) host reference and destroy the
     /// underlying [`HostImpl`], returning its finalized components containing
     /// processing side effects  to the caller as a tuple wrapped in `Ok(...)`.
+    ///
+    /// Use [`Host::can_finish`] to determine before calling the function if it
+    /// will succeed.
     pub fn try_finish(self) -> Result<(Storage, Events), HostError> {
         let events = self.try_borrow_events()?.externalize(&self)?;
         Rc::try_unwrap(self.0)

soroban-env-host/src/test.rs

@@ -24,4 +24,5 @@ mod symbol;
 mod tuple;
 mod vec;
 
+mod finish;
 mod metering_benchmark;

soroban-env-host/src/test/finish.rs

@@ -0,0 +1,31 @@
+use crate::Host;
+
+#[test]
+fn can_finish_on_one_reference() {
+    let host1 = Host::test_host_with_recording_footprint();
+    assert!(host1.can_finish());
+    assert!(host1.try_finish().is_ok());
+}
+
+#[test]
+fn cant_finish_on_multiple_reference() {
+    let host1 = Host::test_host_with_recording_footprint();
+    let host2 = host1.clone();
+    assert!(!host1.can_finish());
+    assert!(!host2.can_finish());
+    assert!(host1.try_finish().is_err());
+    _ = host2;
+}
+
+#[test]
+fn can_finish_on_reduced_to_one_reference_count() {
+    let host1 = Host::test_host_with_recording_footprint();
+    {
+        let host2 = host1.clone();
+        assert!(!host1.can_finish());
+        assert!(!host2.can_finish());
+        _ = host1;
+    }
+    assert!(host1.can_finish());
+    assert!(host1.try_finish().is_ok());
+}