feat: configurable params for built-in workloads

[mrhillsman]

Summary

• Parameterize all five built-in workloads (CPU, memory, disk, database, network) via the existing Params map[string]string mechanism in WorkloadConfig
• Users can now tune workload behavior from config without modifying source — e.g., cpu-load-percent: "50" or rwmixread: "50"
• Follows the getter-with-default pattern already established by chaos workloads (chaos_network.go, chaos_disk.go, etc.)
• All params have sensible defaults matching previous hardcoded values, so existing configs are unaffected

New params by workload

Workload	Params
CPU	cpu-load-percent (100), cpu-method (all)
Memory	memory-percent (80), vm-stressors (1), vm-method (all)
Disk	block-size-rw (4k), block-size-seq (128k), rwmixread (70), numjobs (4), runtime (300)
Database	scale-factor (50), clients (10), duration (300)
Network	parallel-streams (4), test-duration (60)

Resolves #184

[exe-prow-github-app]

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