Patch for custom binary operators that use standard comparison symbols

When a custom binary operator uses a standard comparison operator (`=`, `<>`, `<`, `>`, `<=`, `>=`)
EF Core will generate a pattern that causes a syntax error in PostgreSQL.

Example SQL:
```sql
WHERE x."Inet" < @__inet_1 = TRUE
```

This patch adds logic to check if the custom binary operator is one of the standard
comparison operators. If so, it wraps the expression in parenthesis.

Example SQL (patched):
```sql
WHERE (x."Inet" < @__inet_1) = TRUE
```

While it seems like this could be avoided by having the custom translator construct
a standard `Expression.LessThan(Expression,Expression)`, this causes an error to be
thrown...because the binary operator isn't defined.

Example stack trace:

```
System.InvalidOperationException : The binary operator LessThanOrEqual is not defined
  for the types 'System.Net.IPAddress' and 'System.Net.IPAddress'.
  at System.Linq.Expressions.Expression.GetUserDefinedBinaryOperatorOrThrow(...)
  at System.Linq.Expressions.Expression.GetComparisonOperator(...)
  at System.Linq.Expressions.Expression.LessThanOrEqual(...)
```

Related:

- dotnet/efcore#9143
- #323 (review)

src/EFCore.PG/Query/Sql/Internal/NpgsqlQuerySqlGenerator.cs

@@ -24,6 +24,7 @@
 #endregion
 
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq.Expressions;
 using System.Text.RegularExpressions;
@@ -41,6 +42,15 @@ public class NpgsqlQuerySqlGenerator : DefaultQuerySqlGenerator
     {
         readonly bool _reverseNullOrderingEnabled;
 
+        /// <summary>
+        /// The collection of standard comparison operators.
+        /// </summary>
+        /// <remarks>
+        /// See: https://github.com/aspnet/EntityFrameworkCore/issues/9143
+        /// </remarks>
+        [NotNull] static readonly HashSet<string> StandardComparisonSymbols =
+            new HashSet<string>(new[] { "=", "<>", "<", ">", "<=", ">=" });
+
         protected override string TypedTrueLiteral => "TRUE::bool";
         protected override string TypedFalseLiteral => "FALSE::bool";
 
@@ -322,12 +332,28 @@ public virtual Expression VisitCustomBinary(CustomBinaryExpression expression)
         {
             Check.NotNull(expression, nameof(expression));
 
+            //--------------------------------------------------------------------------------------------
+            // BUG: custom relational operators cause Postgres to throw 42601: syntax error at or near "="
+            // Example:
+            //   source.Where(x => EF.Functions.LessThan(x.Inet, inet));
+            //   WHERE x."Inet" < @__inet_1 = TRUE
+            //
+            // See: https://github.com/aspnet/EntityFrameworkCore/issues/9143
+            //--------------------------------------------------------------------------------------------
+            var operatorIsComparisonSymbol = StandardComparisonSymbols.Contains(expression.Operator);
+
+            if (operatorIsComparisonSymbol)
+                Sql.Append('(');
+
             Visit(expression.Left);
             Sql.Append(" ");
             Sql.Append(expression.Operator);
             Sql.Append(" ");
             Visit(expression.Right);
 
+            if (operatorIsComparisonSymbol)
+                Sql.Append(')');
+
             return expression;
         }