Filter out transactions with negative-id context extension variables during deserialization

[kushti]

While negative ids of context extension variables are allowed in deserializer :

// ContextExtension.serializer

val values = (0 until extSize)
          .map(_ => (r.getByte(), r.getValue().asInstanceOf[EvaluatedValue[_ <: SType]]))

negative id will cause exception during validation due to

// ErgoLikeContext.toSigmaContext

def contextVars(m: Map[Byte, AnyValue]): Coll[AnyValue] = {
      val maxKey = if (m.keys.isEmpty) 0 else m.keys.max  // TODO optimize: max takes 90% of this method
      val res = new Array[AnyValue](maxKey + 1)
      for ((id, v) <- m) {
        res(id) = v
      }
      CSigmaDslBuilder.Colls.fromArray(res)
}

Rejection tests are done in ergoplatform/ergo#2104 .

Happily, a transaction with neg-id context extension variable will be rejected on all the levels (block/mempool/block candidate generation).

Thus shaping semantics of context extension variable ids (prohibition of negative ids) can be done in deserializer in non-breaking way.

Will be done in #951