Convert arr-common chart exportarr property to more generic metrics interface.

[Matthew-Beckett]

This PR serves as the ground work for the rest of the missing arr-stack adjacent applications such as qBittorrent which are not supported by Exportarr to still use arr-common for it's features, but have a more robust interface to push in custom configuration for their required exporter.

Updates to the arr-common Helm library chart to replace the previous exportarr metrics exporter configuration with a more flexible and extensible metrics configuration. The new approach allows for better customization of the metrics exporter container and its integration with the chart, including support for additional arguments, commands, environment variables, and resource settings.

Key changes include:

Metrics Exporter Refactor

• Replaces the exportarr configuration with a new, more generic metrics configuration in values.yaml, values.schema.json, and related templates, allowing for greater flexibility in how metrics exporters are defined and deployed. [1] [2] [3] [4]
• Removes the old arr-common.exportarr.container and port templates, and introduces new arr-common.metrics.container and port templates to support the new configuration. [1] [2]

Template and Service Updates

• Updates deployment and service templates to use the new metrics configuration and container, ensuring the metrics exporter is deployed and exposed according to the new settings. [1] [2]
• Updates the ServiceMonitor template to reference the new metrics port and path, and to conditionally include the metrics endpoint based on the new configuration.

Documentation and Metadata

• Updates the chart version to 0.2.0 and adds relevant keywords for discoverability, such as exporter and metrics. [1] [2]
• Updates the README to document the new metrics configuration options and removes references to the old exportarr configuration. [1] [2] [3]

Schema and Defaults

• Removes the exportarr schema and default values, and adds a comprehensive schema for the new metrics configuration, supporting additional customization (e.g., args, command, envFrom, securityContext, resources, volumeMounts). [1] [2] [3]
• Updates default values to match the new metrics configuration structure.

These changes make the chart more flexible and maintainable for users who need to customize their metrics exporters.