Payload Mediatype and certificate thumbprint annotation constants are not exported for API consumers

[c-grund]

What is not working as expected?

The constants defined in internal/envelope/envelope.go

MediaTypePayloadV1            = "application/vnd.cncf.notary.payload.v1+json"
AnnotationX509ChainThumbprint = "io.cncf.notary.x509chain.thumbprint#S256"

are not exported, and therefore not available for consumers of the sign and verify API. Since these are required values per the specification, programs that wish to work with notation signatures must duplicate these constants.

What did you expect to happen?

These constants should be available through a public notation-go or notation-core-go export

How can we reproduce it?

Attempt to access these required constant values from external code.

Describe your environment

Linux/Go 1.22

What is the version of your notation-go Library?

notation-go v1.1.0
notation-core-go v1.0.2

[Two-Hearts]

That's a good idea.
@c-grund may I ask what you are trying to do that requires the export of these two variables?

[c-grund]

I'm working on set of tools that can sign and verify signed custom oci objects (among other things). We're using notation as a library for one of the signature formats. Despite performing sign/verify through the API, we want to maintain compatibility with the Notation CLI for performing those functions, and thus wish to stick close to the spec. The constants referred to here are marked as MUST items, so we'd rather not redefine them in our code.