Add raw support for GetBlockCertificates.

[MilkywayPirate]

Purpose

Added a new raw command that exposes GetBlockCertificates.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.