Add scalar function call methods to SQLExpressions

[zio0911]

Closes #1657

Summary

Add convenience methods to SQLExpressions for creating scalar function call expressions that support fully qualified (multi-part) function names.

Problem

• RelationalFunctionCall only supports table-valued functions in FROM/JOIN clauses
• Calling scalar functions with schema-qualified names (e.g., other_db.dbo.my_function(arg)) requires manually constructing template strings with {0}, {1} placeholders
• No dedicated API in SQLExpressions for scalar function calls

Solution

Added three new factory methods to SQLExpressions:

• function(Class<T>, String, Object...) - generic scalar function call
• stringFunction(String, Object...) - String-returning function call
• numberFunction(Class<T>, String, Object...) - Number-returning function call

// Before (manual template with placeholders)
Expressions.stringTemplate("other_db.dbo.my_function({0}, {1})", arg1, arg2);

// After (just pass function name and arguments)
SQLExpressions.stringFunction("other_db.dbo.my_function", arg1, arg2);

Supports any naming convention:

• my_function(...) - simple
• schema.my_function(...) - 2-part
• database.schema.my_function(...) - 3-part (cross-database)
• server.database.schema.my_function(...) - 4-part (linked server)

Changes

• SQLExpressions.java - Added function(), stringFunction(), numberFunction() methods and private createFunctionCallTemplate() helper
• SQLExpressionsFunctionTest.java - New test class with 9 test cases covering naming patterns, return types, and SQL contexts (SELECT, WHERE, INSERT VALUES)
• SerializationTest.java - Added 2 scalar function serialization tests with SQLServerTemplates