Create a go struct for Approval Distribution Message

[kishansagathiya]

Issue summary

Translate rust struct for Approval Distribution Message into go.
Make sure that our go struct is same as rust using scale encoding.

• Make sure that both struct produces the same scale encoding.
• Make sure that scale encoding of go struct for Approval Distribution Message can be decoded into rust struct for Approval Distribution Message and they have the same internal properties
• Make sure that scale encodoing of rust struct for Approval Distribution Message can be decoded into go struct for Approval Distribution Message and they have the same internal properties

Other information and links

• Spec for Approval Distribution Message is at https://spec.polkadot.network/chapter-anv#net-msg-approval-distribution
• Description in Node implementer's guide https://paritytech.github.io/polkadot/book/types/overseer-protocol.html#approval-distribution-message
• Rust code here https://github.com/paritytech/polkadot/blob/8eb5efcdaec745e16167043c75e2dd4da958cbc8/node/network/protocol/src/lib.rs#L487

[kishansagathiya]

closed in #3326