Create a go struct for Statement Distribution Message

[kishansagathiya]

Issue summary

Translate rust struct for Statement 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 Statement Distribution Message can be decoded into rust struct for Statement Distribution Message and they have the same internal properties
• Make sure that scale encodoing of rust struct for Statement Distribution Message can be decoded into go struct for Statement Distribution Message and they have the same internal properties

Other information and links

• Spec for Statement Distribution Message is at https://spec.polkadot.network/chapter-anv#net-msg-statement-distribution
• Rust code here https://github.com/paritytech/polkadot/blob/8eb5efcdaec745e16167043c75e2dd4da958cbc8/node/network/protocol/src/lib.rs#L421
• Our scale library https://github.com/ChainSafe/gossamer/tree/development/pkg/scale