Implement dynamic resource manipulation helpers

[fspmarshall]

Provides a set of helper functions to support dynamically handling the validation and storage of common resource types.

This initial implementation covers only those resources supported by the resource-manipulation subcommands of tctl.

Includes a few basic tests to confirm compatibility with equivalent non-dynamic APIs.

note: This is in preparation for #2480, which will require the ability to dynamically initialize a set of resources at startup.

[russjones]

Can you provide some more context on why you want to add support for this?

[fspmarshall]

Can you provide some more context on why you want to add support for this?

Sure thing. I'm currently working on implementing the ability to backup and restore key components of auth server state. Based on my investigation of how state is handled and stored, as well as how a teleport process initializes itself, it seems that the easiest way to do this in a manner that is easy to reason about and extend is to allow a backed to be "bootstrapped" with a collection of resources. This necessitates the ability to accept a dynamic collection of resources, resolve their concrete types, ensure that none of them exist before any of them are written, and then write them all to the backend. There was enough variance in the way different concretely typed APIs in services/local operated (lookup by name vs by type, creation vs upsert, etc.) that it seemed clearer and more consistent to convert all resources into backend.Items, and manage storage/checks that way.

[fspmarshall]

Retracting this PR in favor of adding these helpers alongside the features that require them.