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

New transaction.get_interchain and block.get_interchain call the prime-node
/api/v1/{transaction,block}/{id}/interchain endpoints, returning an
InterchainTrace {block_id, validator_blocks, interchain_transactions}. Adds
VerificationBlock / InterchainTransaction / InterchainTrace dataclasses with
from_dict, exports them, and a from_dict test.

tests/test_client.py

@@ -16,6 +16,7 @@ from prime_sdk import (
     Block,
     DragonchainAPIError,
     DragonchainSDK,
+    InterchainTrace,
     SmartContractCreateRequest,
     Transaction,
     TransactionCreateRequest,
@@ -191,6 +192,42 @@ def test_block_from_dict_nested_header():
     assert len(blk.transactions) == 3
 
 
+def test_interchain_trace_from_dict_nested():
+    # Server shape: validatorBlocks + interchainTransactions arrays, camelCase.
+    raw = {
+        "blockId": "69636602",
+        "validatorBlocks": [
+            {
+                "version": "1",
+                "primeChainId": "zDYr",
+                "primeBlockId": "69636602",
+                "verifierPublicKey": "02c4...",
+                "verifierSignature": "MEUC...",
+            }
+        ],
+        "interchainTransactions": [
+            {
+                "id": 19,
+                "chainId": "1",
+                "transHash": "0xd46e",
+                "validatorBlocks": ["69636602"],
+                "coveredPrimeChainIds": ["zDYr"],
+            }
+        ],
+    }
+    trace = InterchainTrace.from_dict(raw)
+    assert trace.block_id == "69636602"
+    assert len(trace.validator_blocks) == 1
+    assert trace.validator_blocks[0].prime_block_id == "69636602"
+    assert trace.validator_blocks[0].verifier_public_key == "02c4..."
+    assert len(trace.interchain_transactions) == 1
+    assert trace.interchain_transactions[0].id == 19
+    assert trace.interchain_transactions[0].trans_hash == "0xd46e"
+    assert trace.interchain_transactions[0].chain_id == "1"
+    assert trace.interchain_transactions[0].validator_blocks == ["69636602"]
+    assert trace.interchain_transactions[0].covered_prime_chain_ids == ["zDYr"]
+
+
 def test_contract_create_request_omitempty():
     req = SmartContractCreateRequest(
         environment="python3.8",