feat: add Google Calendar connector contract - #20

in API POST /api/v1/proposals added the new `google-calendar` enum value to the `proposal/anyOf[#/components/schemas/ActionPlan]/connector` response property for the response status `200`

oasdiff: couldn't post the review link as a PR comment (HTTP 403). On fork pull requests the token is read-only; otherwise grant 'permissions: pull-requests: write'. The link is still in the job summary.