Go struct of onchain metadata format

[Lucklyric]

Through our current metadata in contract is a generic dict {String: String}.
We need to define a standard format for it.

Here is v-1.0.0 format:

Version: String
Name: String
Description: String
External_url: String?
Image: String?
Attributes: String? // should be a json formatted string
AnimationURL: String?

Use this data structure to implement a mint script with test.