Add iter() method to Extensions (#18887)

## Which issue does this PR close?

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- None

## Rationale for this change

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As a consumer of the `Extension`s, I would like to be able to iterate
all the config extensions present in the extension map. My specific
use-case, is to be able to iterate them all so that I can transparently
dump them in a `HeaderMap` so that I can send them as part of a network
request.

## What changes are included in this PR?

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PR.
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Adds an `iter()` method to the `Extensions` struct

## Are these changes tested?

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yes

## Are there any user-facing changes?

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Users are now able to iterate their custom `ConfigExtension`s

Author: gabotechs

datafusion/common/src/config.rs

@@ -1524,6 +1524,14 @@ impl Extensions {
         let e = self.0.get_mut(T::PREFIX)?;
         e.0.as_any_mut().downcast_mut()
     }
+
+    /// Iterates all the config extension entries yielding their prefix and their
+    /// [ExtensionOptions] implementation.
+    pub fn iter(
+        &self,
+    ) -> impl Iterator<Item = (&'static str, &Box<dyn ExtensionOptions>)> {
+        self.0.iter().map(|(k, v)| (*k, &v.0))
+    }
 }
 
 #[derive(Debug)]
@@ -3037,6 +3045,16 @@ mod tests {
         );
     }
 
+    #[test]
+    fn iter_test_extension_config() {
+        let mut extension = Extensions::new();
+        extension.insert(TestExtensionConfig::default());
+        let table_config = TableOptions::new().with_extensions(extension);
+        let extensions = table_config.extensions.iter().collect::<Vec<_>>();
+        assert_eq!(extensions.len(), 1);
+        assert_eq!(extensions[0].0, TestExtensionConfig::PREFIX);
+    }
+
     #[test]
     fn csv_u8_table_options() {
         let mut table_config = TableOptions::new();