enable multiple providers for the same type

Use Case

When implementing providers for multiple different backend implementations with similar resources (types), having to implement multiple copies of the type is tedious and error prone.

Examples:

• various flavours of mysql (mysql, mariadb, percona)
• implementations of netdev_stdlib types
• several core types (package, user, exec, ...)

Describe the Solution You Would Like

???

Describe Alternatives You've Considered

• using composite namevars to expose the different implementations
• hiding the implementation details completely within a provider
• implement distinct types and provide unified usage through a defined type in puppet

Additional Context

• https://github.com/puppetlabs/puppet-specifications/blob/master/language/resource-api/README.md#multiple-providers-for-the-same-type
• https://groups.google.com/d/msgid/puppet-users/194dfe1f-ff97-4388-92f7-be755de24d7b%40googlegroups.com?utm_medium=email&utm_source=footer
• Originally raised in https://tickets.puppetlabs.com/browse/PDK-530.
• a lesser for of this, transport-specific providers, has been implemented in PDK-1185 and #126

Start with the multiple-providers-for-the-same-type section of the linked resource API specification, then compare the transport-specific provider work in PDK-1185 and #126. The issue does not identify implementation files, tests, or a defined solution, so acceptance criteria and a concrete definition of done need to be established first.