[Feature][Function] support bit_length function (#6140)

support bit_length function like mysql

docs/en/sql-reference/sql-functions/string-functions/bit_length.md

@@ -0,0 +1,54 @@
+---
+{
+    "title": "bit_length",
+    "language": "en"
+}
+---
+
+<!-- 
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+with the License.  You may obtain a copy of the License at
+
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+
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+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# bit_length
+## Description
+### Syntax
+
+'INT bit_length (VARCHAR str)'
+
+
+Return length of argument in bits。
+
+## example
+
+```
+mysql> select bit_length("abc");
++-------------------+
+| bit_length('abc') |
++-------------------+
+|                24 |
++-------------------+
+
+mysql> select bit_length("中国");
++----------------------+
+| bit_length('中国')    |
++----------------------+
+|                   48 |
++----------------------+
+```
+## keyword
+BIT_LENGTH