Extend CLI with custom attestors

[jiparis]

The goal of this feature request is to allow to write custom logic that extracts information from the attestation context and adds it to the final statement. Examples of such attestors exist already in the CLI for getting the runner context or the commit information, including the signature, but it's hardcoded and don't share any common interface.

With this proposal, all those custom extractors will be registered and implement a common interface. This way, the CLI will be able to discover and execute them dynamically.

See witness attestors at https://github.com/in-toto/go-witness/tree/main/attestation