Skip serialization of `Option::None` fields

Currently many structs in the client libraries (algod, kmd, indexer) serialize None fields. Add the proper macro directive from serde to such fields, checking the official Algorand documentation, adding and running tests.

#[serde(skip_serializing_if = "Option::is_none")]

I'd like to pick this one up. Can you provide examples of how to test if serialization is happening properly?

Hi! I thought that instead of adding tests we can use this reference implementation https://github.com/manuelmauro/algorand-oas-generated-v2-clients