COSE_Sign messages (multiple signers) can be read, validated, and created

* [ ] Read and expose metadata for tagged and untagged COSE_Sign messages
* [ ] Validate all signatures with both embedded and detached content, from both formats.
  * [ ] ECDsa
  * [ ] RSASSA-PSS (from [IETF RFC 8230](https://tools.ietf.org/html/rfc8230))
* [ ] Create new embedded- and detached- content signatures.
* [ ] Countersign existing messages.