Add get_ref/get_mut to JSON Writer (#7854)

# Which issue does this PR close?

None

# Rationale for this change

I need access to the writer so that I can flush an external buffer when
bytes are written.

# What changes are included in this PR?

A couple of methods to the JSON writer. These methods already exist on
other writers

# Are these changes tested?

N/A

# Are there any user-facing changes?

Yes, a couple extra methods on the JSON writer.

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: cetra3

arrow-json/src/writer/mod.rs

@@ -413,6 +413,19 @@ where
         Ok(())
     }
 
+    /// Gets a reference to the underlying writer.
+    pub fn get_ref(&self) -> &W {
+        &self.writer
+    }
+
+    /// Gets a mutable reference to the underlying writer.
+    ///
+    /// Writing to the underlying writer must be done with care
+    /// to avoid corrupting the output JSON.
+    pub fn get_mut(&mut self) -> &mut W {
+        &mut self.writer
+    }
+
     /// Unwraps this `Writer<W>`, returning the underlying writer
     pub fn into_inner(self) -> W {
         self.writer