puppetlabs-registry: Spike consolidating defined type with regular type / provider

The difference between the registry::value defined type and the registry_value  type is confusing, as explained in https://tickets.puppetlabs.com/browse/MODULES-8159?focusedCommentId=621978&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-621978

• registry::value is documented at https://github.com/puppetlabs/puppetlabs-registry#registryvalue - parameters key, data, type and value
• registry_value is documented at https://github.com/puppetlabs/puppetlabs-registry#registry_value - parameters path, ensure, type and data

The also have different mechanisms for setting the default value of a key.

Determine whether it is possible to deprecate the defined type in a major version release and/or update documentation to make it clear why the defined type is necessary.

If there's a plan to remove the defined type for clarity, there must be a deprecation warning emitted in a Minor release prior to bumping the Major version of the module.