Remote AppSource

[cohix]

Atmo should be able to connect to a remote AppSource (as defined in #41) that can be relied on as a source of things like a Directive, Wasm Runnables, and more.

This would allow Atmo to be deployed in a completely 'inert' state, and load its configuration from a remote source.

For example, Atmo could load individual Runnable modules on demand when in Headless mode, or it could be configured to use a different Directive based on the hostname of a request, loading on-demand as requests are handled.

Atmo could also use this remote AppSource as a means to deploy new bundle versions or rollback. This could be used to facilitate something akin to GitOps.

I envision a data source being configured by setting anATMO_APP_SOURCE variable with an HTTP or gRPC endpoint and including some form of authentication to ensure the data source can be trusted and vice versa.

[cohix]

Depends on #41

[cohix]

I believe this should be implemented as a gRPC service to start, with a server implementation that can be imported by other projects to provide the actual data.