Add Default instance for UpdateSequenceNumber.

[td202]

Purpose

Add a Default instance for UpdateSequenceNumber. The default value is 1, which is the minimum allowed sequence number.

Changes

• Add Default instance.
• Update grpc-api.

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.

[limemloh]

Please update changelog of concordium-base