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.

block/block.go

@@ -25,3 +25,16 @@ func (bc *BlockClient) Get(ctx context.Context, blockID string) (*models.Block,
 	}
 	return &resp, nil
 }
+
+// GetInterchain traces a block to the validator (verification) blocks that
+// validated it and the public-chain interchain anchors those validator blocks
+// were bundled into.
+func (bc *BlockClient) GetInterchain(ctx context.Context, blockID string) (*models.InterchainTrace, error) {
+	var resp models.InterchainTrace
+	path := fmt.Sprintf("/api/v1/block/%s/interchain", blockID)
+	err := bc.client.Get(ctx, path, &resp)
+	if err != nil {
+		return nil, err
+	}
+	return &resp, nil
+}