Extensibility of the compute platform

[brooke-hamilton]

This document was originally part of #88. It is now in its own PR.

[willtsai]

Good ideas and design principles. I think it could use some clarifications on what exactly is being proposed and respective problems to solve. I see it as two problems:

1. There needs to be a way to extend Radius core so that adding another supported compute platform outside of Kubernetes is possible and easy according to the design principles listed.
2. There is a need provision Radius core resources (namely Environments, Applications, Containers, SecretStores, Gateways) using Recipes.

We should explore what the experience for rad deploy env.bicep and rad deploy app.bicep would look like.

[sylvainsf]

Looks great!