Azure: Add blob collector stub

[suntala]

For #54

Adds an Azure blob cost collector behind -azure.services blob, including metrics wiring that follows the same gatherer pattern as AKS (cost vec collected via Collect rather than separate registration). Azure startup now skips collectors that fail to construct—logging the error and continuing—matching AWS behavior so one bad service does not stop the whole provider.

How to check it locally:

First run the exporter:

go run ./cmd/exporter/exporter.go \
  -provider azure \
  -azure.subscription-id "$(az account show --query id -o tsv)" \
  -azure.services blob \
  -log.level=info \
  -log.type=text

Then scrape the metrics:

curl -s localhost:8080/metrics | grep -E 'azure_blob|collector='

[rooneyshuman]

🚀