GetInterchain: perChain + chains options (default first anchor per chain)

Transaction.GetInterchain and Block.GetInterchain take variadic options
(client.WithPerChain / client.WithChains, re-exported as sdk.WithPerChain /
sdk.WithChains) that map to the new prime-node ?perChain=&chains= query params.
Backward compatible: existing no-option calls get the default (one anchor per
chain). client.InterchainQuery builds the suffix; unit-tested.

block/block.go

@@ -29,9 +29,14 @@ func (bc *BlockClient) Get(ctx context.Context, blockID string) (*models.Block,
 // 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) {
+//
+// By default it returns the first anchor per public chain (anchor proofs are
+// chained, so the earliest per chain is the meaningful one). Use
+// client.WithPerChain / client.WithChains to return more anchors per chain or
+// restrict to specific chains.
+func (bc *BlockClient) GetInterchain(ctx context.Context, blockID string, opts ...client.InterchainOption) (*models.InterchainTrace, error) {
 	var resp models.InterchainTrace
-	path := fmt.Sprintf("/api/v1/block/%s/interchain", blockID)
+	path := fmt.Sprintf("/api/v1/block/%s/interchain%s", blockID, client.InterchainQuery(opts...))
 	err := bc.client.Get(ctx, path, &resp)
 	if err != nil {
 		return nil, err