Fix for more than 50 fields in Postgres

Postgres does not allow functions that have more that 100 arguments.
When using the concat function this limits comparisons for more than 50
fields.
Using `||` for concat like the Oracle variant fixes this.

Author: kindly

data_diff/sqeleton/databases/postgresql.py

@@ -1,3 +1,4 @@
+from typing import List
 from ..abcs.database_types import (
     DbPath,
     JSON,
@@ -92,6 +93,10 @@ def quote(self, s: str):
     def to_string(self, s: str):
         return f"{s}::varchar"
 
+    def concat(self, items: List[str]) -> str:
+        joined_exprs = " || ".join(items)
+        return f"({joined_exprs})"
+
     def _convert_db_precision_to_digits(self, p: int) -> int:
         # Subtracting 2 due to wierd precision issues in PostgreSQL
         return super()._convert_db_precision_to_digits(p) - 2