Add COSE support

[SteveLasker]

A prototype of notation cose support has been completed.

This tracks:

• Complete remote signing support: veraison/go-cose #9
• Notary Spec support of COSE #48
• Add support for COSE, through the veraison/go-cose project

Other tasks:

• Implement signing artifacts with COSE as signing envelope (repo: notation-core-go/notation-go) #287
• Implement verification of signatures enclosed in COSE envelope (repo: notation-core-go/notation-go) #288
• Update notation CLI spec to support COSE as signing envelope (repo: notation) #289
• Implement notation CLI to support COSE as signing envelope (repo: notation) #290
• Implement plugin signing artifacts with COSE as signing envelope (repo: notation-core-go/notation-go) #291
• Implement plugin verification of signatures enclosed in COSE envelope (repo: notation-core-go/notation-go) #292
• Refactor code to support multiple envelopes

[shizhMSFT]

The COSE support should be through notation-core-go, where notation-core-go implements COSE support through the veraison/go-cose project.