CLI command to list available providers

[tegefaulkes]

Specification

Right now we only have github as a provider. But there is no way to know what providers are available unless you look at the source code.

To this end we need to add a polykey identities providers command (name pending) that will list all supported providers.

Functionality may need to be added to Polykey to support this.

We can also look into adding a list of providers to the help text of the command?

Additional context

Related: #40

Tasks

1. Add a CLI command to list supported providers.

[CMCDragonkai]

Because provider integration will later be dynamic with plugin support (MatrixAI/Polykey#166 and https://github.com/MatrixAI/Polykey/issues?q=is%3Aissue+is%3Aopen+providers), it does need to be an actual command itself.

So yes pk identities providers is a useful way of listing all providers, including which ones are natively integrated and which ones are plugins.

Plugins are likely be wasm integrated - this makes it easy for us to call code in a safe way (sandboxing) and allow contributors to write in whatever language they please.