Implement COUNT and COUNT(DISTINCT) aggregation operations

  Add comprehensive COUNT aggregation support addressing issue com-lihaoyi#95:

  - Add AggAnyOps.scala with count/countDistinct for Expr[T] types
  - Add countBy/countDistinctBy methods to AggOps.scala
  - Full cross-database compatibility (PostgreSQL, MySQL, SQLite, H2, MS SQL)
  - Proper NULL handling (COUNT ignores NULL values, COUNT(*) counts all)
  - Support for expressions: map(...).count, map(...).countDistinct
  - Multi-dialect SQL generation with proper operator precedence
  - Comprehensive test coverage including edge cases and complex expressions

  Features:
  - .countBy(_.column) generates COUNT(column) SQL
  - .countDistinctBy(_.column) generates COUNT(DISTINCT column) SQL
  - .count/.countDistinct on mapped expressions for advanced scenarios
  - Works with arithmetic expressions, filters, group by, and joins
  - Consistent API design matching existing aggregation functions

Author: vim89

scalasql/test/src/ConcreteTestSuites.scala

@@ -11,7 +11,8 @@ import operations.{
   ExprBlobOpsTests,
   ExprMathOpsTests,
   DbCountOpsTests,
-  DbCountOpsOptionTests
+  DbCountOpsOptionTests,
+  DbCountOpsAdvancedTests
 }
 import query.{
   InsertTests,
@@ -85,6 +86,7 @@ package postgres {
   object ExprMathOpsTests extends ExprMathOpsTests with PostgresSuite
   object DbCountOpsTests extends DbCountOpsTests with PostgresSuite
   object DbCountOpsOptionTests extends DbCountOpsOptionTests with PostgresSuite
+  object DbCountOpsAdvancedTests extends DbCountOpsAdvancedTests with PostgresSuite
 
   object DataTypesTests extends datatypes.DataTypesTests with PostgresSuite
 
@@ -136,6 +138,7 @@ package hikari {
   object ExprMathOpsTests extends ExprMathOpsTests with HikariSuite
   object DbCountOpsTests extends DbCountOpsTests with HikariSuite
   object DbCountOpsOptionTests extends DbCountOpsOptionTests with HikariSuite
+  object DbCountOpsAdvancedTests extends DbCountOpsAdvancedTests with HikariSuite
 
   object DataTypesTests extends datatypes.DataTypesTests with HikariSuite
 
@@ -185,6 +188,7 @@ package mysql {
   object ExprMathOpsTests extends ExprMathOpsTests with MySqlSuite
   object DbCountOpsTests extends DbCountOpsTests with MySqlSuite
   object DbCountOpsOptionTests extends DbCountOpsOptionTests with MySqlSuite
+  object DbCountOpsAdvancedTests extends DbCountOpsAdvancedTests with MySqlSuite
   // In MySql, schemas are databases and this requires special treatment not yet implemented here
   // object SchemaTests extends SchemaTests with MySqlSuite
   object EscapedTableNameTests extends EscapedTableNameTests with MySqlSuite
@@ -235,6 +239,7 @@ package sqlite {
   // object ExprMathOpsTests extends ExprMathOpsTests with SqliteSuite
   object DbCountOpsTests extends DbCountOpsTests with SqliteSuite
   object DbCountOpsOptionTests extends DbCountOpsOptionTests with SqliteSuite
+  object DbCountOpsAdvancedTests extends DbCountOpsAdvancedTests with SqliteSuite
   // Sqlite doesn't support schemas
   // object SchemaTests extends SchemaTests with SqliteSuite
   object EscapedTableNameTests extends EscapedTableNameTests with SqliteSuite
@@ -290,6 +295,7 @@ package h2 {
   object ExprMathOpsTests extends ExprMathOpsTests with H2Suite
   object DbCountOpsTests extends DbCountOpsTests with H2Suite
   object DbCountOpsOptionTests extends DbCountOpsOptionTests with H2Suite
+  object DbCountOpsAdvancedTests extends DbCountOpsAdvancedTests with H2Suite
 
   object DataTypesTests extends datatypes.DataTypesTests with H2Suite
   object OptionalTests extends datatypes.OptionalTests with H2Suite
@@ -341,6 +347,7 @@ package mssql {
   object ExprMathOpsTests extends ExprMathOpsTests with MsSqlSuite
   object DbCountOpsTests extends DbCountOpsTests with MsSqlSuite
   object DbCountOpsOptionTests extends DbCountOpsOptionTests with MsSqlSuite
+  object DbCountOpsAdvancedTests extends DbCountOpsAdvancedTests with MsSqlSuite
 
   object DataTypesTests extends datatypes.DataTypesTests with MsSqlSuite