Aggregate impact values across all the resources when calling estimate on the CLI

[jnioche]

Calling estimate with the CLI returns the impacts values for each resource. It would be useful to also return an aggregate of all the counts to give an overview of the overall impact.

[demeringo]

We already have a build summary function that aggregaes impacts. https://github.com/Boavizta/cloud-scanner/blob/72413a8b682d695349fd6c7a889554fe8aa3a935/cloud-scanner-cli/src/impact_provider.rs#L63 At the moment it is only used when producing metrics but we could re-use it with json output.

I imagine 2 possibilities:

• add the summary at the end of the json output (always)
• update cli flags or add API route to retrieve only the summary