Added default set of features to Agent Configuration

[oliveromahony]

Proposed changes

Adding default set of feature flags to enable refactoring of Agent Configuration properly.

This is the basics for the design of a feature flag management system for the NGINX Agent. The system enables dynamic toggling of features based on configurations provided via multiple sources, including CLI parameters, environment variables, configuration files, and via protobuf gRPC messages. Feature flags control the execution paths within the application, allowing for flexible and controlled feature management.

Feature Overview:

• configuration - Full read/write management of configurations toggled by DataPlaneConfig ConfigMode
• certificates - Inclusion of public keys and other certificates in the configurations toggled by DataPlaneConfig CertMode
• connection - Sends an initial connection message reporting instance information on presence of Command ServerConfig Host and Port
• file-watcher - Monitoring of file changes in the allowed directories list and references from product configs. When enabled, may trigger DP => MP synchronisation of files
• agent-api - Future Agent REST API
• metrics - Full metrics reporting.
• metrics-host - All host-level metrics.
• metrics-container - Future container metrics reporting.
• metrics-instance - Instance level metrics, will have various categories in the future

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13