feature(datepicker): Create a CDK version of the datepicker

It would be nice to create a cdk-datepicker that captures the interaction pattern of the various sub-components that make up the datepicker, but provides no concrete UI. It would be up to users (of which the Angular Material date-picker would be one) to build the actual UI and styling.

This is not something that we have any immediate plans to do, but I'm creating this issue to document it as something to consider in the future.

No files, tests, or entry points are named. Start by reviewing the existing Angular Material datepicker and Angular CDK architecture to define the intended headless interaction model; the issue does not specify an implementation boundary or completion criteria.

Written by the indexing model from the issue text.