enhancement(topology): Add internal-only topology (DRAFT)

[bruceg]

This is a proof-of-concept spike to test the feasibility of adding a separate, secondary, internal-only topology within Vector.

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[lukesteensen]

Very neat to see that this works! I think the next steps to explore this path would be finding anything that could conflict across the two, such as internal metrics.

That said, from our discussion it seems like the idea of attaching namespaces to individual components within one topology is potentially interesting to explore as well. That would involve some of the same challenges (e.g. internal metrics), but also figuring out a story for fault isolation and shutdown ordering.

[bruceg]

Superseded by #18919