Support required vs nullable

[bhelx]

No description provided.

[bhelx]

@zshipko could i get a hand with the tests here? it seems i'm upsetting some macro that i just don't know how to mentally decode.

[zshipko]

The issue is that we end up with a field like:

        #[serde(rename = "value")]
        #[serde(with = "Base64Standard")]
        pub value: Option<Vec<u8>>,

where Base64Standard encoding doesn't work because it expects Vec<u8> - maybe we could work around this using a newtype with its own serialize/deserialize implementations. I need to look into other options, but we could also just replace the #[serde(with = "Base64Encoder")] with another encoder that works on Option values.

[bhelx]

Closing in favor of #15