bevy_reflect: Add `ReflectSerializerProcessor`

[aecsocket]

NOTE: This is based on, and should be merged alongside, https://github.com/bevyengine/bevy/pull/15482. I'll leave this in draft until that PR is merged.

Objective

Equivalent of https://github.com/bevyengine/bevy/pull/15482 but for serialization. See that issue for the motivation.

Also part of this tracking issue: https://github.com/bevyengine/bevy/issues/15518

This PR is non-breaking, just like the deserializer PR (because the new type parameter P has a default P = ()).

Solution

Identical solution to the deserializer PR.

Testing

Added unit tests and a very comprehensive doc test outlining a clear example and use case.