Switch DSSE provider to go-securesystemslib

[adityasaky]

Summary

Switches from using github.com/in-toto/in-toto-golang/pkg/ssl to github.com/secure-systems-lab/go-securesystemslib/dsse.

Ticket Link

N/A.

Release Note

Uses new provider for DSSE.

[lukehinds]

Thanks @adityasaky could you give a little more context, for example this is more optimised code, the ssl modules will be deprecated and are these at parity around features. Just need to understand any possible impacts.

[adityasaky]

Whoops, meant to include a link: in-toto/in-toto-golang#122.

The code at github.com/secure-systems-lab/go-securesystemslib/dsse is exactly the same as the prototype that is currently in in-toto-golang. It was originally written with in-toto in mind, given ITE-5 (https://github.com/in-toto/ITE/blob/master/ITE/5/README.adoc), but we decided it made sense for it to live independently for use in other projects (see: in-toto/in-toto-golang#110).

[lukehinds]

Thanks @adityasaky , will override CI as its not related to this change (its' the aws plugin code).