Add infrastructure to ensure our stable types remain stable

[flub]

noq generally is not 1.0 yet, so we do not guarantee much API stability. However some types are directly exported in iroh. We need to have infrastructure to protect against mistakes.

The types re-exported are already clearly documented in iroh/src/endpoint/quic.rs. We need to make sure we also clearly annotate this here to make sure they are not accidentally changed.

• Each type needs to be clearly commented/documented that it is considered stable.
• We need CI to check those types remain API stable.