Add configurable cache TTL and jitter settings.

Problem

Currently, the image existence cache has a hardcoded TTL of 24 hours with a 3-hour jitter. This can be problematic for users who:

• Need to adjust cache duration based on potential artifact cleanup policies
• Want to disable caching entirely
• Need to tune jitter settings to prevent cache stampede in their environment

Solution

Added two new configuration options:

• cacheTtlMinutes (default: 1440 - 24 hours): Controls how long image existence information is cached
• cacheJitterMaxMinutes (default: 180 - 3 hours): Controls the maximum random time added to prevent cache stampede

Both settings are configurable via the .k8s-image-swapper.yaml config file. Once this gets reviewed I'll make some updates to the helm-chart to support these new configuration values.

Fixes #843