Create deterministic decoder for cose signed documents

[stevenj]

Summary

Create a deterministic decoder for the catalyst signed document type

Description

Acceptance criteria

• Decoder has context to allow decoding to be strict (fails if not deterministic) log warning if not deterministic, or silently accept.
• The existing interface/API for Catalyst Signed Documents should not change, or if it does, it should be because it was unavoidable, and the changes must be as small as possible.
• To use the helpers created in Helper to decode and check for deterministic encoding with minicbor. #333
• To be tested against data encoded by the Encoder in Implement true minicbor encoder for catalyst signed documents. #332

Notes

This task should not be started until both #333 and #332 are complete.

[no30bit]

I uncovered a bug. Currently, when validating ContentType::Cbor we use

minicbor::decode::<minicbor::data::Token>(content)

However, the Token doesn't represent a valid CBOR value, unlike serde_json::Value and serde_json::RawValue. It's lazy and pretty much only checks the head datatype byte (see the source here).
That means that an arbitrary byte sequence may follow and we wouldn't even know.
The solution I propose is using the minicbor::Decoder::skip function in a loop until the end of input. In that case we won't unnecessarily allocate and also eagerly decode the data.

CC @stevenj @Mr-Leshiy

[Mr-Leshiy]

@no30bit yes, you are right, will to fix it. Could you create a bug ticket for it.
But trying to solve this issue, need to be careful, maybe we will dont want to change current implementation, in any case need to revise this validation.