Latest `catalyst-signed-doc-spec/v0.2.0` for `catalyst-signed-doc` crate

[Mr-Leshiy]

Summary

Use the latest release catalyst-signed-doc-spec/v0.2.0 for the catalyst-signed-doc crate.
Integrating the latest updates among with the voting documents: Contest Ballot and Contest Ballot Checkpoint.

Description

• Make and use the latest catalyst-signed-doc-spec/v0.2.0 release for the catalyst-signed-doc crate.
• Update the existing tests if they would fail according to the latest specifications.
• Add integration tests for the new document types Contest Ballot and Contest Ballot Checkpoint.