Add function definition support for the data mapper

[nipunayf]

Purpose

Extend the function definition functionality to the data mapper by implementing the data mapper definition for the getSourceCode, getNodeTemplate, and functionDefinition APIs. We can reuse the function form in the UI by specifying "DATA_MAPPER_DEFINITION" in the node template, while keeping the rest of the implementation unchanged.

Remarks: We can create a more abstract layer for the function form to promote code reusability. We can explore this approach as we introduce more functions to the diagram to capture more requirements.