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.

client/interchain_test.go

@@ -0,0 +1,22 @@
+package client
+
+import "testing"
+
+func TestInterchainQuery(t *testing.T) {
+	cases := []struct {
+		name string
+		opts []InterchainOption
+		want string
+	}{
+		{"none", nil, ""},
+		{"perChain1", []InterchainOption{WithPerChain(1)}, "?perChain=1"},
+		{"perChain0 (all)", []InterchainOption{WithPerChain(0)}, "?perChain=0"},
+		{"chains", []InterchainOption{WithChains("1", "0")}, "?chains=1,0"},
+		{"both", []InterchainOption{WithPerChain(2), WithChains("1", "0")}, "?perChain=2&chains=1,0"},
+	}
+	for _, c := range cases {
+		if got := InterchainQuery(c.opts...); got != c.want {
+			t.Errorf("%s: InterchainQuery = %q, want %q", c.name, got, c.want)
+		}
+	}
+}