improvement(vars/longevityPipeline.groovy): Make use of separator plugin

[Lakshmipathi]

This PR modifies longevityPipeline.groovy to better organize and document pipeline parameters.

• Grouped parameters into logical sections using separator for better UI.
• Existing pipeline functionality remains the same.
• Added few missing description part.
• Added separator for following sections headers. Cloud Provider Configuration,Stress Test Configuration,ScyllaDB Configuration,Provisioning Configuration,Post Behavior Configuration,SSH Configuration,Manager Configuration,Email and Test Configuration,Kubernetes Configuration,Miscellaneous Configuration,BYO ScyllaDB Configuration

The difference between existing UI and UI can be found in below screenshot. Job UI itself can be found in below URL

Testing

• [x ] https://jenkins.scylladb.com/job/scylla-staging/job/LakshmipathiGanapathi/job/test%20parameter%20separator/job/pline/24/rebuild/parameterized [x] https://argus.scylladb.com/tests/scylla-cluster-tests/8981862b-bb61-4dfa-b564-97a65eceb5e1 [x] https://jenkins.scylladb.com/job/scylla-staging/job/LakshmipathiGanapathi/job/longevity-100gb-4h-test/37/parameters/

PR pre-checks (self review)

• [ x] I added the relevant backport labels
• [ x] I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)