public ChatRequestBuilder function(String functionName) {
		request.addFunction(functionName);
		return this;
	}
functions the model
@param functionName The name of the function to be called. Must be a-z, A-Z, 0-9,
or contain underscores and dashes, with a maximum length of 64.
@return builder
src/main/java/org/mvnsearch/chatgpt/model/completion/chat/ChatRequestBuilder.java