feat: add Scanner API endpoints

[derrickobedgiu1]

Implements the three Power BI Scanner API endpoints for retrieving workspace metadata including dataset schema and RLS role definitions.

// Requires AdminServicePrincipal connector
$scan    = PowerBI::postWorkspaceInfo([$workspaceId], datasetSchema: true);
$status  = PowerBI::getScanStatus($scan->id);   // poll until $status->isSucceeded()
$result  = PowerBI::getScanResult($scan->id);

Requires the tenant setting "Allow service principals to use read-only admin APIs" to be enabled. datasetSchema / datasetExpressions additionally require "Enhanced metadata scanning".