[Feature] CostGuard scorer skeleton

[davidbreitgand]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR is the first of a four-PR series that lands the CostGuard scorer plugin (see README for rationale, algorithm, and design).

Each PR compiles, runs, passes its own tests, and adds a bounded, well-scoped increment.

PR 1 (this PR): Plugin skeleton, config, factory, runner registration.
Lands the framework plumbing: PluginType, Config with full validation, factory, etc., and runner registration. Score returns a neutral 0.5 for every model, which is a valid "no opinion" state that keeps the scorer registerable and pipeline-safe. windowDuration parameter is parsed and validated but not consumed until PR 4 (marked with TODO in the code)

Future PRs outline (for convenience of reference and scope of work:

PR2 and PR3 are two halves of the same algorithm that must land together (eventually) to be useful.

PR 2: Exploitation branch (rank + sigmoid scoring).
Introduces the deterministic ranking half of the algorithm: reads each model's CostDigest from the datalayer, computes its intermediate rank, and applies a temperature-scaled sigmoid for obtaining the score in (0, 1). Models without cost history continue to receive the neutral 0.5. PR2 is independently reviewable but not deployable in isolation since the scorer requires the exploration branch from PR 3 to bootstrap its own cost data collection.

PR 3 — Exploration branch (epsilon-greedy).
Exploration branch (epsilon-greedy).
Adds the random exploration branch that drives sample collection on under-explored models. Together with PR 2, this completes the scoring algorithm. After PR 3 lands, the scorer is usable but not production-grade.

PR 4 — Epoch/window semantics (extractor owns the window).
Extends the requestcostmetadata extractor to archive and reset its per-model t-digest each windowDuration, so published snapshots reflect only the current epoch. Removes WindowDuration from the scorer's Config (the extractor becomes the single source of truth) and updates both plugin READMEs.

Refs #214

Release note (write NONE if no user-facing change):

NONE

[davidbreitgand]

/cc @shmuelk

[shmuelk]

/lgtm
/approve