apierror: simplify extraction of custom errors

[shollyman]

Within apierror, the ErrDetails type holds decoded values from an rpc status:
https://pkg.go.dev/github.com/googleapis/gax-go/v2/apierror#ErrDetails

Custom messages such as service-specific custom error types end up in the Unknown slice of ErrDetails, which is a slice of empty interface types. It would be beneficial to provide a utility mechanism for extracting a message of a given type from the ErrDetails.

By way of example, the bigquerystorage service has a StorageError type:
https://pkg.go.dev/google.golang.org/genproto/googleapis/cloud/bigquery/storage/v1#StorageError

Being able to more easily extract this from APIError would be lovely. Perhaps something like this might make sense?

if msg := apiErr.Details().ExtractDetail(&storagepb.StorageError{}); msg != nil {
  if se, ok := msg.(*storagepb.StorageError); ok {
    // do something with the storage error
  }
}

Also happy to make the change if this seems reasonable.

cc @noahdietz @quartzmo