Fix Block model to match server: nest header with camelCase keys

The block endpoint returns block id / prev / timestamp nested under a
"header" object with camelCase keys (blockId, dcId, prevId, prevProof,
timestamp) and a proof of just {proof}. The previous flat snake_case
Block fields never matched the response and always deserialized empty.
Add a BlockHeader dataclass, nest it in Block, make proof.scheme
optional, and cover it with a unit test. Verified live against a dev
chain.

prime_sdk/__init__.py

@@ -45,6 +45,7 @@ from .contract import ContractClient
 from .errors import DragonchainAPIError, DragonchainError
 from .models import (
     Block,
+    BlockHeader,
     BlockProof,
     GrpcConnectionInfo,
     ListResponse,
@@ -121,6 +122,7 @@ __all__ = [
     "TransactionTypeClient",
     # models
     "Block",
+    "BlockHeader",
     "BlockProof",
     "GrpcConnectionInfo",
     "ListResponse",