add GetInterchain: trace a transaction/block to validator blocks + interchain anchors

New Transaction.GetInterchain and Block.GetInterchain call the prime-node
/api/v1/{transaction,block}/{id}/interchain endpoints, returning an
InterchainTrace {blockId, validatorBlocks, interchainTransactions}. Adds local
VerificationBlock / InterchainTransaction / InterchainTrace model types.

transaction/transaction.go

@@ -52,6 +52,20 @@ func (tc *TransactionClient) Get(ctx context.Context, transactionID string) (*mo
 	return &resp, nil
 }
 
+// GetInterchain traces a transaction to the validator (verification) blocks that
+// validated its prime block and the public-chain interchain anchors those
+// validator blocks were bundled into. If the transaction is still pending (not
+// yet in a block) the trace's slices are empty.
+func (tc *TransactionClient) GetInterchain(ctx context.Context, transactionID string) (*models.InterchainTrace, error) {
+	var resp models.InterchainTrace
+	path := fmt.Sprintf("/api/v1/transaction/%s/interchain", transactionID)
+	err := tc.client.Get(ctx, path, &resp)
+	if err != nil {
+		return nil, err
+	}
+	return &resp, nil
+}
+
 func (tc *TransactionClient) List(ctx context.Context) (*models.ListTransactionsResponse, error) {
 	var resp models.ListTransactionsResponse
 	path := "/api/v1/transaction/"