Invoke-ScriptAnalyzer: Print summary only once per invocation

[MatejKafka]

PR Summary

Similarly to the previous -EnableExit PR, this PR moves the summary printed when -ReportSummary is passed to EndProcessing, so that it's only printed once, instead of once for every value from the pipeline.

Imo, users are likely to be interested in the summary from all of the piped files, not for each file separately. Since the message is printed to the host (so the caller has no easy way to capture it), I don't believe moving it to be a breaking change.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.