feat(extensions): show required materials

[migmartri]

Shows the list of required materials that an extension is subscribed to. This can be useful so the user knows what kind of material they need to setup in their contract to be picked up by the extension.

I do not indicate that an extension is triggered on attestation because I don't think it's that relevant for the configuration of the extension.

Additionally, I updated the API to leave room to future extension types

┌─────────────────┬─────────┬──────────────────────────────────────┬───────────────────────────────────────────────────────────┐
│ ID              │ VERSION │ MATERIAL REQUIREMENT                 │ DESCRIPTION                                               │
├─────────────────┼─────────┼──────────────────────────────────────┼───────────────────────────────────────────────────────────┤
│ dependencytrack │ 0.2     │ SBOM_CYCLONEDX_JSON, CONTAINER_IMAGE │ Send CycloneDX SBOMs to your Dependency-Track instance    │
├─────────────────┼─────────┼──────────────────────────────────────┼───────────────────────────────────────────────────────────┤
│ smtp            │ 0.1     │                                      │ Send emails with information about a received attestation │
├─────────────────┼─────────┼──────────────────────────────────────┼───────────────────────────────────────────────────────────┤
│ oci-registry    │ 0.1     │                                      │ Send attestations to a compatible OCI registry            │
└─────────────────┴─────────┴──────────────────────────────────────┴───────────────────────────────────────────────────────────┘

closes #170
refs #38

[danlishka]

LGTM