ESQL: Add url_decode function (elastic#133494)

- Add url_decode as a snapshot function.
- Move common logic to both encode/decode to a parent class

Author: mouhc1ne

docs/reference/query-languages/esql/_snippets/functions/description/url_decode.md

@@ -2501,3 +2501,39 @@ ROW u = ["hello elastic!", "a+b-c%d", "", "!#$&'()*+,/:;=?@[]"] | EVAL u = URL_E
 u:keyword
 ["hello+elastic%21", "a%2Bb-c%25d", "", "%21%23%24%26%27%28%29*%2B%2C%2F%3A%3B%3D%3F%40%5B%5D"]
 ;
+
+url_decode sample for docs
+required_capability: url_decode
+
+// tag::url_decode[]
+ROW u = "https%3A%2F%2Fwww.example.com%2Fpapers%3Fq%3Dinformation%2Bretrieval%26year%3D2024%26citations%3Dhigh" | EVAL u = URL_DECODE(u)
+// end::url_decode[]
+;
+
+// tag::url_decode-result[]
+u:keyword
+https://www.example.com/papers?q=information+retrieval&year=2024&citations=high
+// end::url_decode-result[]
+;
+
+url_decode mixed functions tests
+required_capability: url_decode
+
+FROM employees 
+| WHERE emp_no == 10001 
+| EVAL a = TRIM(URL_DECODE(first_name))
+| EVAL b = URL_DECODE(TO_LOWER(first_name))
+| KEEP a,b;
+
+a:keyword | b:keyword
+Georgi    | georgi 
+;
+
+url_decode mixed input tests
+required_capability: url_decode
+
+ROW u = "%21%23%24%26%27%28%29*%2B%2C%2F%3A%3B%3D%3F%40%5B%5D" | EVAL u = URL_DECODE(u);
+
+u:keyword
+"!#$&'()*+,/:;=?@[]"
+;