Support web-sdk v7

[limemloh]

Purpose

This will enable support of newer versions of the web-sdk (v7) without any breaking changes.

Changes

Replace CcdAmount with broader type HasMicroCcd which works with both old and new version of web-sdk.

Context: Since v7 of web-sdk removes methods on CcdAmount it cannot be used with the current released version of the wallet api (helpers), since this uses the old type which require these methods. Since the wallet does not rely on these methods and only on the internal field microCcdAmount of CcdAmount we can support both versions by reflecting this in the type.

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.

[soerenbf]

Looks good :) Could you bump version to 2.8.1 as well and make the release? If you don't have permission to release on NPM, I can also do that.

[shjortConcordium]

**** you
(No longer seems relevant)