Add step function for Tuya Zigbee curtains

[zhangshen001]

Check all that apply

Type of Change

• WWST Certification Request
  • If this is your first time contributing code:
    • I have reviewed the README.md file
    • I have reviewed the CODE_OF_CONDUCT.md file
    • I have signed the CLA
  • I plan on entering a WWST Certification Request or have entered a request through the WWST Certification console at developer.smartthings.com
• Bug fix
• New feature
• Refactor

Checklist

• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have verified my changes by testing with a device or have communicated a plan for testing
• I am adding new behavior, such as adding a sub-driver, and have added and run new unit tests to cover the new behavior

Description of Change

Summary of Completed Tests

[CLAassistant]

All committers have signed the CLA.

[vangoran]

Let's add '_' into value and make same style with other static variables

[vangoran]

跟已有的全局变量保持一致，采用驼峰命名法 + 前缀‘_’