Keylime allowlist signing and verification improvements

Allow more modern key types for allowlist signing, and improve allowlist verification with optional transparency log integration (for example, with Sigstore).

- Keylime Enhancement Proposal: https://github.com/keylime/enhancements/pull/65
- Primary contact (assignee): @mbestavros
- Enhancement target (which target equals to which milestone):
- Alpha release target (x.y)
- Beta release target (x.y)
- Stable release target (x.y)

Start by reviewing the Keylime Enhancement Proposal in pull request 65. Use it to determine the intended modern key types and the scope of optional transparency-log verification, such as Sigstore integration. The issue is complete when those requirements and the alpha, beta, and stable targets are defined.

Written by the indexing model from the issue text.