docs(proposals): Add proposal to extend pluggable scanner spec

[Vad1mo]

Adds a new proposal under proposals/new/extend-pluggable-scanner-spec.md that proposes a backward-compatible v1.3 revision of the pluggable scanner spec (current: v1.2.0, June 2024) and a set of additive Harbor core changes to turn the scanner subsystem into a general-purpose image-analysis plugin framework with chaining.

The proposal opens the capability namespace (Dockerfile reconstruction, lint, helm.analyze, license, secret, malware, policy, attestation, etc.) and lets adapters declare consumes_capabilities so Harbor can build a DAG of analyzers — output of one feeding the next.

Related

• Depends on / unblocks: Proposal: Image Optimization & Recommendation Engine #282 (Image Optimization & Recommendation Engine)
• Builds on: Pluggable Image Vulnerability Scanning (2019)
• Builds on: SBOM Generation and Scan (2024)
• Spec repo: https://github.com/goharbor/pluggable-scanner-spec (issue to be opened there as Track 1 once this proposal is accepted)