Add helper method alias to JwtPayload similar to ShopifyApp::JWT

[zzooeeyy]

Description

• Partially for: https://github.com/Shopify/develop-app-access/issues/289

Add helper methods to JwtPayload with names similar to ShopifyApp JWT to make deprecation migration easier.

  def shopify_domain
    @payload && ShopifyApp::Utils.sanitize_shop_domain(@payload["dest"])
  end

  def shopify_user_id
    @payload["sub"].to_i if @payload && @payload["sub"]
  end

  def expire_at
    @payload["exp"].to_i if @payload && @payload["exp"]
  end

How has this been tested?

Unit tested the methods to return expected values

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[paulomarg]

Just a nit.

[paulomarg]

Maybe we should list the methods here or just give an indication of what their purpose is?