Refactor function encode_attributes

[Patrik-Stas]

In file encoding.rs function pub fn encode_attributes(attributes: &str) -> VcxResult<String> takes &str as argument - it then tries to parse it to Hashmap<String, serde_json::Value>.

Task:

• change the function input and output types to be less generic than just &str and String - for example, it could look like

pub fn encode_attributes(attributes: CredentialAttributes) -> VcxResult<CredentialAttributesEncoded>

and hence making the function declaration communicate more about the input it requires and output it's producing.

Obivously, this will have impact on all of the code calling the function, so you will have to update that appropriately. The main place where encode_attributes is being called from. You could transitively continue applying this typing improvements on calling functions.

Additionally, you can try to refactor the function implementation itself as it's quite long.

[Patrik-Stas]

This issue has been picked up by Styvane in #778