Feat(dot/parachain): Add CompactStatement type

[axaysagathiya]

Changes

• introduced CompactStatement and compactStatementInner types. We will only deal with CompactStatement in other codebases. compactStatementInner is only used for encoding/decoding of CompactStatement, which is why it's not exported.
• implemented MarshalSCALE and UnmarshalSCALE methods to have custom encoding/decoding logic for CompactStatement.
• also updated the sign logic of statementVDT to encode the data properly before signing.

Tests

go test ./dot/parachain/types/... -v

Issues

Closes #4422