Add ra_system_sup:child_spec/1 builder

[the-mikedavis]

This allows other OTP applications to start a system under their own supervision tree, rather than the dynamic ra_system_sup supervisor. This makes for finer control over lifecycle, for example making it possible for a consuming application to naturally terminate itself if the system goes down, if the system is a critical part of the application.

The child_spec/1 builder is pretty trivial

child_spec(#{name := Name} = Config) ->
    #{id => Name,
      type => supervisor,
      start => {?MODULE, start_link, [Config]}}.

This change also moves the ownership of the {'$ra_system', Name} from ra_systems_sup into a separate server so that it is set and cleared consistently.

See #585

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