Use encoded/decoded string when serializing / deserializing

[UMR1352]

Description of change

Calling serde::deserialize should work the same as SdJwt::parse and serde::serialize should work as SdJwt::presentation but RN they just serialize or deserialize the inner structure of SdJwt. This PR fixes this behaviour.

Type of change

• Bug fix (a non-breaking change which fixes an issue)

• I have followed the contribution guidelines for this project

• I have performed a self-review of my own code

• I have commented my code, particularly in hard-to-understand areas

• I have made corresponding changes to the documentation

• I have added tests that prove my fix is effective or that my feature works

• I have checked that new and existing unit tests pass locally with my changes

• I have updated the CHANGELOG.md, if my changes are significant enough

[abdulmth]

Looks good, it would be nice if we have a test for that

[UMR1352]

On second thought I removed the implementation of Serialize and Deserialize from SdJwt. The problem is that they depend on the representation of the implementer of Serializer and Deserializer. For instance when using serde_json, an sd_jwt encoded value is represented by a JSON string which requires data to be enclosed in double quotes (") and we don't want that. @abdulmth @eike-hass @wulfraem any thoughts?