Add @Singular support for Optional types

[velo]

Summary

This PR adds support for using @Singular with Optional types in @Builder.

Changes

• Added JavacJavaUtilOptionalSingularizer for javac compiler support
• Added EclipseJavaUtilOptionalSingularizer for Eclipse compiler support
• Added comprehensive test cases for Optional singularization

Behavior

For a field annotated with @Singular Optional<T>, the builder generates:

• A setter method accepting the unwrapped type T
• A clear method to reset the optional
• Build code that wraps the value in Optional.ofNullable() or returns Optional.empty()

Example

@Builder
class Example {
    @Singular
    private Optional<String> name;
}

// Generated methods:
builder.name("John")    // Sets the value
builder.clearName()     // Clears the value
builder.build()         // Returns Optional.ofNullable(value) or Optional.empty()

Test Plan

• Compiles successfully with ant dist
• Added test cases in test/transform/resource/
• Tests should be run with ant test

Fixes #3976

[ExE-Boss]

Since this is using Optional​::ofNullable(…), the …$set subfield is redundant.

[velo]

Since this is using Optional​::ofNullable(…), the …$set subfield is redundant.

done