Add Vendor to Vulnerability Scanners

...te/20200622070606_add_vendor_to_vulnerability_scanners.rb

...00622070620_add_limit_to_vulnerability_scanners_vendor.rb

db/structure.sql

ee/app/models/vulnerabilities/scanner.rb

ee/changelogs/unreleased/210327-add-scanner-vendor-field.yml

ee/spec/factories/vulnerabilities/scanners.rb

ee/spec/models/vulnerabilities/scanner_spec.rb