TASK: just changing sparsity to optional (QueryTypeConfig field in encryption_fields.idl)

CONSIDERATIONS:

• I had originally NOT made sparsity optional because the documentation specified that sparsity should have a default value (0, at least for now).
• Something can't be optional & have a default value. 
• If I make sparsity optional, we need to enforce the default value somewhere else
  • I considered doing this in resolved_encryption_info.cpp, but since the query info is always a const, this wasn't possible

So, making it optional is no problem as long as we enforce the default value somewhere else (i.e., not resolved_encryption_info), which I'm sure is possible.