Add fuzzing for BlockLoader endpoints

[luckychess]

Description of the Change

Fuzzing for block loader endpoints (retrieveBlock and retrieveBlocks).

Benefits

Another endpoint is covered by fuzzing.

Possible Drawbacks

Hard to support until fuzzing in CI is not ready.

Usage Examples or Tests

cmake -DCMAKE_C_COMPILER=/usr/local/opt/llvm/bin/clang -DCMAKE_CXX_COMPILER=/usr/local/opt/llvm/bin/clang++  -DFUZZING=ON ..
make retrieve_block_fuzz
make retrieve_blocks_fuzz

[l4l]

ServerWriter cannot be mocked :( I've tried with ServerWriterInterface, but it cannot be casted safely (e.g only via reinterpret_cast. Here's an example of such mock (path test/module/vendor/grpc/grpc_mocks.hpp):

class MockServerWriter
    : public grpc::ServerWriterInterface<iroha::protocol::ToriiResponse> {
  MOCK_METHOD1(Write, void(iroha::protocol::ToriiResponse));
  MOCK_METHOD2(Write,
               bool(const iroha::protocol::ToriiResponse &,
                    grpc::WriteOptions));
  MOCK_METHOD0(SendInitialMetadata, void());
  MOCK_METHOD1(NextMessageSize, bool(uint32_t *));
};

But it's not really a good idea to use it. Need to think about that issue

[igor-egorov]

LGTM

Works inside run-iroha-dev.sh container

[l4l]

Should be static, I guess
Moreover some calls probably should be mocked (particularly the ones that return boolean)

[luckychess]

I don't think it should be static - we pass a non-const raw point to it and have no idea what will happen with it inside.
I'm not sure what call do you mean - the only one which returns boolean is Write with 2 args but we are not interested in it.