Handle Slack Approve/Reject clicks on hosted-signup approval alerts

Request

• Base URL
  https://api.definite.app (the document declares no server; this is the origin it was published from)
• Auth
  none declared
• TypeScript
  https://skmtc.dev/definite/apis/defapi/docs/onprem/v1/slack/interactions/post.ts

  A function with typed inputs. Its response is checked against the schema.