Add a command to export configuration

Problem/Motivation

Part of #3582246: [meta] CLI in Core community initiative. Referenced from theCLI in Core Initiative spreadsheet.

Exporting configuration is important for deployment scenarios and site source control.

Steps to reproduce

Working on a Drupal Core CLI Command issue > Test Commands

Ensure that the configuration sync directory is setup in environment.

Export configuration as a zip file via the user interface, and extract to a different directory for comparison.

Export the configuration using dr config:export.

Do a diff between the YAML files from UI export and the CLI export to ensure there are no differences.

Make a change in the user interface.

Export the configuration using the CLI to ensure that change is exported to YAML.

Proposed resolution

To be determined.

Remaining tasks

User interface changes

To be determined.

Introduced terminology

To be determined.

API changes

To be determined.

Data model changes

To be determined.

Release notes snippet

To be determined.