Add ACH instruction

curl --request POST \
  --url https://sandbox.featherhq.com/funding/linking \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <x-api-key>' \
  --data '{
  "account_id": "<string>",
  "account_number": "<string>",
  "bank_account_type": "Checking",
  "bank_name": "<string>",
  "routing_number": "<string>"
}'

{
  "account_id": "<string>",
  "ach_id": "<string>",
  "ach_instruction_name": "<string>",
  "requires_manual_verification": true,
  "status": "<string>"
}