Subscription Manager 1.0.0-beta6

Phase 3 of the stable-release roadmap (#3616939): features.

- #3618739: Charge-based connector mode with a local billing cycle
  engine. New subscription_charge entity (one record per period, the
  idempotency key), monthly/yearly cycles with month-end day clamping,
  immediate prorated upgrades, downgrades at the anchor, refunds, and
  configurable zero-amount period records. Connectors declare
  supported_modes; charge-based ones implement
  ChargeBasedConnectorInterface.
- #3616769: Connector chooser in the subscribe flow when multiple
  connectors offer visible plans, with configurable order and
  per-connector label/description overrides (restricted HTML allowed).
  The default_connector setting is replaced by the chooser_connectors
  sequence: the first installed entry is the default and recommended
  connector (update 10019 migrates existing sites). The subscribe-url
  API gains an additive multi-connector response shape.
- #3615735: Usage rating seam. Sites implement UsageRaterInterface as
  tagged services; the cycle engine bills rated usage in arrears
  alongside the advance base fee and records the lines on the charge.
- #3618741: Tax/invoice seam. TaxInvoiceServiceInterface (calculate,
  recordCharge, recordRefund) with a null default; bridges replace the
  service to plug in a real tax engine. Charges gain an
  invoice_reference field; exclusive tax is added to charges, inclusive
  tax recorded; refunds produce credit-note references.
- #3621282: Subscriber billing-history page at
  /user/{user}/billing-history with invoice link-outs, plus an
  account-menu link.

Also: default admin views for all three entities, Views data for
subscription charges, and charge failure records now preserve the rated
and tax line audit trail.

Contains updates 10015-10020. Change records:
https://www.drupal.org/list-changes/subscription_manager