feat(type,jer): implement Encode/Decode on serde_json::Value

[mkatychev]

Structs like this will faild to derive since serde_json::Value (despite being used in the jer module) does not implement Encode/Decode/AsnType:

#[derive(serde::Serialize, serde::Deserialize, Debug, PartialEq, Eq, Clone, AsnType)]
pub struct Foo {
    inner: serde_json::Value
}

Used choice type to implement said traits for serde_json::Value.

[mkatychev]

I'm fairly new/clueless as to the ASN.1 standard and am not sure if there anything from X.697 that can be used in this approach though the CHOICE type seemed best to mirror the serde_json::Value enum.

[mkatychev]

part of motivation is that json canonicalization is not available in no_std