Support starts_with (str, prefix) function (#2813)

Support starts_with function

docs/documentation/cn/sql-reference/sql-functions/string-functions/starts_with.md

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+
+# starts_with
+## description
+### Syntax
+
+`BOOLEAN STARTS_WITH (VARCHAR str, VARCHAR prefix)`
+
+如果字符串以指定前缀开头，返回true。否则，返回false。任意参数为NULL，返回NULL。
+
+## example
+
+```
+MySQL [(none)]> select starts_with("hello world","hello");
++-------------------------------------+
+| starts_with('hello world', 'hello') |
++-------------------------------------+
+|                                   1 |
++-------------------------------------+
+
+MySQL [(none)]> select starts_with("hello world","world");
++-------------------------------------+
+| starts_with('hello world', 'world') |
++-------------------------------------+
+|                                   0 |
++-------------------------------------+
+```
+##keyword
+STARTS_WITH