[FEATURE] Improve API coverage to show change %

[dblock]

Is your feature request related to a problem?

The API coverage workflow comments with something like "API specs implemented for 247/649 (38%) APIs.". This doesn't say what changed.

What solution would you like?

That message to say "API specs implemented for 247/649 (38%) APIs. No change." or "API specs increased from 235/649 to 247/649 (from 37% to 38%) APIs.".

What alternatives have you considered?

The message doesn't matter. We want to know whether the coverage is increasing or decreasing.

Do you have any additional context?

Change coverage workflows in https://github.com/opensearch-project/opensearch-api-specification/tree/main/.github/workflows.

[dblock]

Closing via #297.