Adding Contains ESQL String function (#133016)

Adding ES:QL Contains function and tests

Author: mjmbischoff

docs/changelog/133016.yaml

@@ -0,0 +1,5 @@
+pr: 133016
+summary: Adding Contains ESQL String function
+area: ES|QL
+type: feature
+issues: []

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

@@ -1,6 +1,7 @@
 * [`BIT_LENGTH`](../../functions-operators/string-functions.md#esql-bit_length)
 * [`BYTE_LENGTH`](../../functions-operators/string-functions.md#esql-byte_length)
 * [`CONCAT`](../../functions-operators/string-functions.md#esql-concat)
+* [`CONTAINS`](../../functions-operators/string-functions.md#esql-contains)
 * [`ENDS_WITH`](../../functions-operators/string-functions.md#esql-ends_with)
 * [`FROM_BASE64`](../../functions-operators/string-functions.md#esql-from_base64)
 * [`HASH`](../../functions-operators/string-functions.md#esql-hash)

docs/reference/query-languages/esql/_snippets/functions/examples/contains.md

@@ -21,6 +21,9 @@ mapped_pages:
 :::{include} ../_snippets/functions/layout/concat.md
 :::
 
+:::{include} ../_snippets/functions/layout/contains.md
+:::
+
 :::{include} ../_snippets/functions/layout/ends_with.md
 :::