[collector] Add Keda support

[matheus-meneses]

Hi guys,

I would like to propose a new feature and a breaking change to improve the autoscaling capabilities of the OpenTelemetry Collector Helm chart.

1. Add KEDA support:
   KEDA (Kubernetes Event-driven Autoscaling) reached General Availability in August 2023, and its ecosystem is growing rapidly. Given its importance in the Kubernetes autoscaling landscape, I believe it's crucial to add support for KEDA in our Collector's Helm chart.

2. Restructure autoscaling configuration:
   To accommodate both HPA (Horizontal Pod Autoscaler) and KEDA, I propose a breaking change in the autoscaling configuration. This change will provide a clear separation between the two autoscaling methods and improve the overall structure of the configuration.

Proposed changes:
a. Move autoscaling.behavior to autoscaling.hpa.behavior
b. Introduce an autoscaling.mode field to select between hpa (default) and keda
c. Create a new configuration section autoscaling.keda to support KEDA-specific settings

Example of the proposed structure:

autoscaling:
  enabled: true
  mode: hpa | keda
  minReplicas: 1
  maxReplicas: 10
  targetCPUUtilizationPercentage: 80
  targetMemoryUtilizationPercentage: 80
  hpa:
    behavior: {}
  keda:
    # ... KEDA-specific settings

Originally posted by @matheus-meneses in #1475