When AzureSentinelLogger is resolved from the string callback name
"azure_sentinel", it is constructed with no arguments, so audit_stream_name is
always None and resolved_audit_stream_name fell back to the standard
resolved_stream_name. Audit logs then ingested into the access-log DCR stream
whose schema is built from StandardLoggingPayload, so Azure Monitor Logs
Ingestion silently dropped the audit-specific columns and audit rows arrived
effectively empty.
Add an AZURE_SENTINEL_AUDIT_STREAM_NAME env var fallback in __init__, mirroring
the AZURE_SENTINEL_STREAM_NAME idiom already used for the standard stream, so
audit logs can target a separate DCR stream without a custom callbacks file.
