"Send Feedback" UI: show "supported services" list

## Problem

It is not clear what services are supported by AWS Toolkit. Customers may think that a service is missing in the AWS Explorer because of region differences, but actually it's just that AWS Toolkit doesn't yet support it.

## Proposal

1. in the Toolkit "Send Feedback" UI, show a brief message like `AWS Toolkit currently supports X services. Are there services you would really like to see in AWS Toolkit?`
2. customer can expand the services list in the "Send Feedback" UI and choose any of them.
3. when customer clicks `Send` on the "Send Feedback" UI, the message includes a list of the requested services

Start by locating the Toolkit's "Send Feedback" UI and the code that assembles feedback messages. The work is done when the UI explains the supported-service scope, lets customers expand and select services, and includes their selections when they send feedback.

Written by the indexing model from the issue text.