Add kubeconfig-style configuration file support

[CharlieTLe]

Implements kubeconfig-style configuration for managing multiple Cortex clusters, similar to how kubectl manages Kubernetes clusters.

Features:

• Multiple contexts, clusters, and users configuration
• Config file location: ~/.cortextool/config (default)
• Global --config flag to specify custom config file path
• Global --context flag to override current-context
• Config management subcommands (view, get-contexts, use-context, etc.)
• Automatic TLS certificate and authentication credential management
• Configuration precedence: CLI flags > Env vars > --context > current-context > Defaults

Configuration structure:

• contexts: Bind clusters and users together
• clusters: Define Cortex endpoints and TLS settings
• users: Define tenant IDs and authentication credentials
• current-context: Default context to use

Commands added:

• cortextool config view - Display merged configuration
• cortextool config get-contexts - List all contexts
• cortextool config current-context - Show active context
• cortextool config use-context - Switch contexts
• cortextool config set-context - Create/update context
• cortextool config set-cluster - Create/update cluster
• cortextool config set-credentials - Create/update credentials
• cortextool config delete-context - Remove context

Updated commands:

• All API commands (rules, alertmanager, etc.) now support config file defaults
• Address and tenant ID flags are now optional when config file is present

Documentation:

• Added CONFIG.md with complete usage guide and examples
• Added cortextool.example.yaml as reference
• Updated README.md with quick start guide

[friedrichg]

It's a good idea. Thanks.