RFC for pluggable logging model

Per #106, this is to track the need for an RFC that would create a pluggable provider model for logging. Quoting me from that thread:

@PowerShell/powershell-committee discussed this one in detail today. We believe that there's massive value in figuring out how to tee logs off to remote logging providers, but a bunch of the semantics in this RFC are specific to Splunk.

Instead, we think an RFC should be authored that builds a provider/plugin model for allowing multiple remote logging providers. This RFC should also give consideration to whether local logging targets should be treated the same way (e.g. if you want to exclusively log to a remote target, maybe in serverless/stateless scenarios). There should also be a consideration of whether we should have something like a Get-PSLog or Write-PSLog that are agnostic to logging providers.

The PowerShell Team has a strong interest in picking this up, but we have no idea when we can get to it right now. We only know that it should be a flexible, pluggable model.

Start by reading issue #106 and the PowerShell Committee discussion quoted here to understand the existing Splunk-specific semantics. Draft an RFC defining a flexible provider/plugin model, including remote and local targets and provider-agnostic logging operations; done means the proposed model addresses these scenarios and is ready for community feedback.