v0.3.0 - Budgets and Price Quotes

Breaking changes

• vngcloud.Config is an opaque handle built by vngcloud.NewConfig with
  WithRegion, WithProjectID, WithIAMUser, and the existing options.
  vngcloud.NewClient(ctx, cfg) takes that Config and no options. Every
  client built from one Config shares one login and project lookup.
• The ClientOption alias is now LoadOption.
• A POST or PATCH is retried only after a 429 or a failed connection,
  never after a 5xx or a network error that may have reached the server.
  IsRetryable follows the same rule.
• APIError.Code and ErrorCode now return a numeric envelope code as
  decimal text, such as "400", for every service. They returned ""
  before.

Highlights

• New billing package: budgets, budget thresholds, alert history,
  current-period cost, cost explorer, and balances. Budget and threshold
  create, update, and delete are the first write APIs.
• New pricing package: GetQuote prices a resource before it is created
  and places no order.
• ErrInvalidInput for a missing required field, a malformed ID, or a bad
  date, returned before any request.
• vngcloud.Ptr builds pointer fields for partial updates.
• EndpointOverrides.Billing moves the dashboard billing gateway.
• make live-write runs a gated live budget write test.

Known limits

• The cost explorer and alert history models follow the console's field
  names but have not been checked against a live response with data yet.