feat(unstable): opt-in Charon CDF-writer integration

[nithinb]

Why?

Charon is a new CDF-writer service (POC, changing contract). Instead of extractors writing timeseries straight to CDF, they can register their task config once and stream data through Charon, which decorates/routes/shapes items (Kuiper expressions, regex space routing) and forwards to CDF with the caller's own bearer token. We want extractors built on this library to opt into Charon without disturbing the existing direct-to-CDF path, and without committing the POC's unstable wire contract to the stable public API.

What?

Opt-in support, entirely under Cognite.Extractor.Utils.Unstable.Charon. No stable type changes behavior or signature. Disabled by default (cdf-writer.enabled: false) → direct-to-CDF path is byte-for-byte unchanged.

• ICharonClient / CharonClient — typed client for /cdfwriter/setup and /cdfwriter/payload; the single isolation seam for the wire format. Reuses the existing bearer-token HTTP flow; logs only status/counts, never tokens or item bodies.
• CharonWriter — façade to declare the task set (CharonTask.SpaceRouting / .Custom), register it at startup (SetupAsync), and create upload queues.
• CharonUploadQueue / CharonDatapointsUploadQueue — IUploadQueue<T> writers that chunk each task's items to CDF limits and map results to QueueUploadResult<T>.
• OrderedStringMap — preserves space_routing mapping order (first regex match wins).
• Config / DI — cdf-writer section on ConnectionConfig, DestinationUtilsUnstable.AddCharonWriter, gated in the runtime. Schema + docs updated; version → 1.46.0.

Key decisions:

1. Fallback = config switch only. On Charon 5xx/unreachable the upload fails like any CDF error; flip enabled: false to use the direct path.
2. Setup runs once at startup. Charon's store is in-memory, so a Charon restart mid-run causes payload failures until the extractor restarts.
3. Typed builders + raw JsonElement escape hatch for task registration and item bodies.
4. Datapoints = one reading per payload item (no pivot); throughput caveat documented.

Carry-forward: regex anchoring for space routing is not yet frozen Charon-side (full-match vs substring) — client sends patterns verbatim, noted in builder docs.

How?

Extractor authors:

var writer = Provider.GetRequiredService<CharonWriter>();
writer.RegisterTask(CharonTask.SpaceRouting(
    "route-ts", CharonDestinations.TimeSeriesCreate,
    new[] { new KeyValuePair<string,string>("abcd.*", "space-1") },
    defaultSpace: "space-default"));
await writer.SetupAsync(GetExtractorVersion(), token);

var dps = writer.CreateDatapointsQueue("dps", TimeSpan.FromSeconds(5), 10000, logger);
dps.Enqueue("ts-external-id", new Datapoint(DateTime.UtcNow, 42.0));

Config (connection.yml):

cdf-writer:
  enabled: true
  base-url: "https://charon.example.com"

Testing

19 new tests: wire-contract (camelCase + mapping-order), client (200/207/422/401/5xx), queues (chunking, datapoint expansion, DI). Full Unstable suite 154/154 green; dotnet build of the solution clean.

Note: test project targets net9.0; this machine has net8/net10 runtimes, so tests ran with DOTNET_ROLL_FORWARD=LatestMajor.