Add `register_bundle()` to `ErasedTemplate`

[villor]

Objective

ErasedTemplate does not currently provide a way to get bundle information, which can be useful for a number of reasons.

• In my case, I needed it for reconciliation experiments
• Could also help with dynamic bundle insertion during spawn_scene.

Solution

• Add register_bundle() to ErasedTemplate.

[villor]

https://github.com/bevyengine/bevy/pull/20158