Add get_bit to BooleanBufferBuilder (#693)

* Add get_bit to BooleanBufferBuilder

* fix clippy

arrow/src/array/builder.rs

@@ -311,6 +311,11 @@ impl BooleanBufferBuilder {
         }
     }
 
+    #[inline]
+    pub fn get_bit(&mut self, index: usize) -> bool {
+        bit_util::get_bit(self.buffer.as_slice(), index)
+    }
+
     #[inline]
     pub fn is_empty(&self) -> bool {
         self.len == 0
@@ -2647,6 +2652,31 @@ mod tests {
         assert_eq!(buffer.finish().as_slice(), &[0b01010110_u8, 0b1011100_u8]);
     }
 
+    #[test]
+    fn test_bool_buffer_builder_get_first_bit() {
+        let mut buffer = BooleanBufferBuilder::new(16);
+        buffer.append_n(8, true);
+        buffer.append_n(8, false);
+        assert!(buffer.get_bit(0));
+    }
+
+    #[test]
+    fn test_bool_buffer_builder_get_last_bit() {
+        let mut buffer = BooleanBufferBuilder::new(16);
+        buffer.append_n(8, true);
+        buffer.append_n(8, false);
+        assert!(!buffer.get_bit(15));
+    }
+
+    #[test]
+    fn test_bool_buffer_builder_get_an_inner_bit() {
+        let mut buffer = BooleanBufferBuilder::new(16);
+        buffer.append_n(4, false);
+        buffer.append_n(8, true);
+        buffer.append_n(4, false);
+        assert!(buffer.get_bit(11));
+    }
+
     #[test]
     fn test_boolean_array_builder_append_slice() {
         let arr1 =