Handle attachments in a generic way

[jakubkoci]

We should add something to handle attachments in a generic way (ACA-Py has it). Data could be base64 or json (or others but the first two are most important)

Currently we assume base64 everywhere and use the data.base64, but it could also be in json form.

// new
const data = request.data.get() // could be either json or base64, but it doesn't matter

// old
const base64 = request.data.base64 // could be null
const data = base64decode(base64)

const data = request.data.json

Should be updated in packages/core/src/decorators/attachment/Attachment.ts

Originally posted by @TimoGlastra in #531 (comment)

[jakubkoci]

That could be good, at least for base64 and json. However, if I look at Aries RFC 0017: Attachments it can also contain links. Should we call some HTTP request in that case or just return the link?

[TimoGlastra]

@morrieinmaas and @Annelein will be working on this

[TimoGlastra]

Fixed