Adding pubspec support for survey validator

[eliasyishak]

This PR adds a new key to the json file to restrict certain dash tools from receiving surveys. It also uses the typical dart console structure so that we can import package:unified_analytics to get the list of valid dash tools

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read the Flutter Style Guide recently, and have followed its advice.
• [x] I signed the CLA.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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