Add support for custom variables in configuration files

[butschster]

This PR adds support for defining custom variables directly in configuration files that can be referenced throughout the context definitions.

Read more about variables here

Changes

• Added ConfigVariableProvider to store and manage custom variables
• Created VariablesParserPlugin to extract variables from configuration files
• Added VariablesBootloader to register the plugin with proper dependencies and integrate the custom variables provider

Example

variables:
  version: 1.0.0
  environment: development
  project_name: Context Generator

documents:
  - description: 'Project documentation'
    outputPath: docs/{{environment}}/{{project_name}}-{{version}}.md
    sources:
      - type: text
        content: |
          # {{project_name}} v{{version}}
          Environment: {{environment}}

Schema update

Updated the JSON schema to include the new variables section.

Closes #143