Update/change fields of a reflected/dynamic type

## What problem does this solve or what need does it fill?

I can only get a `&mut dyn Reflect` from `field_mut` from `DynamicTupleStruct` (and similar) which I can modify, but e.g. changing the field from being a `DynamicStruct` to a `DynamicEnum` isn't possible.

## What solution would you like?

Add the option to replace fields

## What alternative(s) have you considered?

I currently have no idea of how to do that externally..

## Additional context

Might be easier when unifying fields (https://github.com/bevyengine/bevy/issues/14847#issuecomment-2301674088) ?

Start by reading the reflection APIs around DynamicTupleStruct and field_mut, then review the discussion linked from issue #14847 about unifying fields. Determine how a field could be replaced with another reflected dynamic type, and consider the public API and tests needed to demonstrate that replacement works.

Written by the indexing model from the issue text.