ci: create .duvet/config.toml

[camshaft]

Description of changes:

This change updates our duvet scripts to use the new config format from: awslabs/duvet#152.

$ duvet report
  Extracting requirements
   Extracted requirements from 5 specifications 31ms
    Scanning sources
     Scanned 976 sources 132ms
     Parsing annotations
      Parsed 2426 annotations 70ms
     Loading specifications
      Loaded 43 specifications 33ms
     Mapping sections
      Mapped 418 sections 2ms
    Matching references
     Matched 8589 references 60ms
     Sorting references
      Sorted 8589 references 83ms
     Writing .duvet/reports/report.html
       Wrote .duvet/reports/report.html 25ms

Call-outs:

Note that the latest version of duvet has not been published, so CI will fail until that happens.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.