feat(go): Generate nil-safe getter methods

This adds support for nil-safe getter methods for each generated type. Callers can more easily access nested properties by chaining together getter method calls instead of a long series of != nil checks.

For example,

Before

response, err := client.Users.Get(ctx, "john.doe")
if err != nil {
  return nil, err
}
if response.User != nil && response.User.Address != nil {
  return response.User.Address.Street, nil
}

After

response, err := client.Users.Get(ctx, "john.doe")
if err != nil {
  return nil, err
}
if address := response.GetUser().GetAddress(); address != nil {
  return address.Street, nil
}

fern-api / fern

feat(go): Generate nil-safe getter methods #5137