getInterchain: perChain + chains options (default first anchor per chain); bump 1.5.0

transaction.getInterchain / block.getInterchain take an optional
{ perChain?, chains? } that maps to prime-node's ?perChain=&chains= params.
Default (no options) returns one anchor per chain. Shared buildInterchainQuery
helper + InterchainOptions type, exported; jest-tested.

README.md

@@ -145,6 +145,24 @@ await sdk.transactionType.delete('my-type');
 const block = await sdk.block.get('block-id');
 ```
 
+### Interchain trace
+
+`transaction.getInterchain` / `block.getInterchain` trace a prime block to the
+public-chain anchors covering it. By default they return the **first anchor per
+chain** (anchor proofs are chained, so the earliest per chain is the meaningful
+one). Tune with options:
+
+```typescript
+// Default: first anchor per chain (ETH, BTC, …)
+const trace = await sdk.block.getInterchain('42');
+
+// Up to 3 anchors per chain
+await sdk.block.getInterchain('42', { perChain: 3 });
+
+// All anchors, only the ETH-mainnet chain ("1"); "0" = BTC
+await sdk.block.getInterchain('42', { perChain: 0, chains: ['1'] });
+```
+
 ### Proof Measure (public, unauthenticated)
 
 `proof-measure` is a separate, **public, unauthenticated** Dragonchain service