Make dotnet new post actions extendable via components

The post action managed by `dotnet new` are already managed by components, however the interface to implement the processor is not public.

- move `IPostActionProcessor` interface to `Abstractions` to make it public, so the template authors can implement it.

The assembly with implemented interface may be installed using [components](https://github.com/dotnet/templating/issues/2942).

Also other host developers may consider using the same interface to implement extendable set of post actions for the host.

Consider moving [PostActionDispatcher](https://github.com/dotnet/sdk/blob/main/src/Cli/Microsoft.TemplateEngine.Cli/PostActionDispatcher.cs) to `Microsoft.TemplateEngine.Edge` so the implementation can be shared for all the hosts.