[XTA-15079] Add SPOG support for ?o= routing in httpPath

[samikshya-db]

Summary

Ports the SPOG (Single Panel of Glass) routing fix from
databricks/databricks-jdbc#1316
to the Node.js driver. SPOG replaces workspace-specific hostnames with
account-level vanity URLs. When httpPath carries ?o=<workspaceId>,
endpoints that don't include the workspace in their URL path need the
workspace conveyed via the x-databricks-org-id header instead.

Changes

• Parse ?o=<digits> out of httpPath in DBSQLClient.connect() and
  stash the org-id as x-databricks-org-id on a new
  ClientConfig.customHeaders field. A user-supplied customHeaders
  entry (case-insensitively keyed) takes precedence.
• Telemetry POST (DatabricksTelemetryExporter) spreads
  config.customHeaders into the outgoing headers; auth headers still
  win on collision.
• Feature-flag GET (FeatureFlagCache) does the same.
• ConnectionOptions.customHeaders is exposed so callers can also
  inject their own out-of-band headers if needed.

What does NOT get the header

OAuth / OIDC token requests use openid-client's private HTTP transport
and never see customHeaders. This matches the JDBC PR: account-level
OAuth endpoints reject x-databricks-org-id with HTTP 400.

What is NOT needed vs JDBC

• httpPath property parser fix — Node.js passes options.path
  through to the driver unmodified; there is no split-on-= parser to
  break.
• Warehouse ID regex fix — this driver uses Thrift only and never
  extracts the warehouse ID into a SEA JSON body. The existing ?o=
  parser (extractWorkspaceId) already stops at ?.
• DBFS Volume client header injection — the driver exposes no
  Volume API surface.

Test plan

Unit tests added:

• buildCustomHeaders parses ?o= into x-databricks-org-id
• No header when ?o= is absent and no user-supplied customHeaders
• User-supplied customHeaders is preserved alongside parsed org-id
• User-supplied x-databricks-org-id (any case) wins over ?o=
• Non-numeric o=<value> does not inject a header
• End-to-end DBSQLClient.connect() populates
  config.customHeaders['x-databricks-org-id'] from the path
• DatabricksTelemetryExporter attaches config.customHeaders to
  the POST headers
• Auth headers still override customHeaders on key collision
• No header is attached when config.customHeaders is empty
• FeatureFlagCache.fetchFeatureFlag attaches customHeaders to
  the GET headers

This pull request and its description were written by Isaac.

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