[RFC - Server] Refining the service builder API (#1859)

* RFC draft.

* Fix checkbox.

* Briefly describe the strategy in the `Overview` section.

* Add setter snippet

* Dive deeper into alternative mechanisms for scoping plugins.

* Add more changes.

* Fix snippet.

* Elaborate on type erasure.

* Remove dangling footnote.

* Fix common type.

* Add more caveats.

* Update the first approach for lazy type erasure