Add REST accessor for decoded WAGED assignments

[LZD-PratyushBhatt]

The controller persists the WAGED baseline and best possible assignments through ZkBucketDataAccessor, which GZIPs the serialized record and splits it across numbered bucket ZNodes. Reading those ZNodes over the existing /zookeeper API therefore returns opaque compressed chunks, and the propertyStore API cannot reach them at all because ASSIGNMENT_METADATA lives at the cluster root rather than under PROPERTYSTORE. The only other assignment API, /partitionAssignment, recomputes a what-if placement and never reads what was actually persisted.

Add WagedAssignmentAccessor, which reassembles the buckets, decompresses, and deserializes each resource assignment server side:

GET /clusters/{clusterId}/wagedAssignment/bestPossible
GET /clusters/{clusterId}/wagedAssignment/baseline

Supported query params:
format=IdealStateFormat (default) | CurrentStateFormat
resources, instances, partitions - comma separated allowlists, since
these payloads are large on real clusters
includeMetadata (default true) - the persisted write version, its ZK
mtime, and the bucket metadata, so callers can tell how fresh the
decoded assignment is

A missing assignment returns 404 rather than an empty body, so callers can distinguish "WAGED never persisted here" from "assignment is empty".

The path layout and the per-resource decode are exposed from AssignmentMetadataStore instead of being duplicated in helix-rest, so the reader cannot drift from the writer.

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