Credential and Presentation attributes mime-type adjustments

[bobozaur]

The attributes (CredentialAttr and PresentationAttr) could benefit from handling the value and mime-type pair in a different manner. Right now both fields are deserialized and handling would have to be done based on whether a MimeType is present or not.

A better idea would be to use an AttrValue #[serde(flatten)] enum with two variants:

• Plain { value: String }
• Encoded { value: String, mime_type: MaybeKnown<MimeType> }

Note the MaybeKnown enum use, which would also be a beneficial addition in case we get a mime_type we don't recognize.

Having the attribute value deserialized this way would make handling it easier since we can match and deduce that:

• if it's Plain, it's just a string,
• if it's Encoded, it's a base64 encoded string that then has to be processed according to the MimeType.

Steps:
- [ ] Wrap MimeType in MaybeKnown
- [ ] Create and use the AttrValue enum in both attribute types.

[SumantxD]

@bobozaur I would like to work on this issue could you assign it to me plz.

[SumantxD]

@bobozaur can you help me locate Attribute struct inside the file structure, i'am struggling to find it

[bobozaur]

@SumantxD My bad, the struct rename was already done but I was looking at an older branch when drafting this issue. I updated the issue description to match the main branch.

[SumantxD]

@bobozaur I have tried solving the issue here is the PR #831