feat: support for creating upload v2 notifier jobs (#5729)

# Description

This PR introduces support for creation of upload_v2 notifier jobs
required for batching of staging files

**Key Changes:**
- Files are grouped into v1 and v2 based on the presence of
BytesPerTable and a feature flag.
- Extracted common logic for v1 and v2 paths into separate methods
- For now, v2 notifier jobs include only one staging file per job,
similar to v1. This is temporary — batching logic will be implemented in
the next PR.
- We will no longer track the processing status (success/failure) of
staging files in v2.
- v2 jobs will always force creation of load files (see
state_generate_load_files.go).

## Linear Ticket


https://linear.app/rudderstack/issue/WAR-461/enable-creation-of-upload-v2-notifier-jobs

## Security

- [x] The code changed/added as part of this pull request won't create
any security issues with how the software is being used.

Author: shekhar-rudder

services/notifier/notifier.go

@@ -35,8 +35,9 @@ const (
 type JobType string
 
 const (
-	JobTypeUpload JobType = "upload"
-	JobTypeAsync  JobType = "async_job"
+	JobTypeUpload   JobType = "upload"
+	JobTypeUploadV2 JobType = "upload_v2"
+	JobTypeAsync    JobType = "async_job"
 )
 
 type JobStatus string