Reflection should be fully optional and toggleable at a `bevy` level

[alice-i-cecile]

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

Reflection results in significant increases to compile times and binary size, and in many projects, is only useful as a development tool.

What solution would you like?

bevy_reflect should be an optional dependency for all Bevy crates whenever possible,
and we should have a reflect feature flag on bevy itself that controls this.

Because of our setup, this will need to be mirrored to bevy_internal.

What alternative(s) have you considered?

We could gate this behind the debug feature flag, as they serve similar roles, but not debug / dev applications of reflection exist.

Additional context

I encountered this when trying to fix #20038. Because a top-level feature flag does not exist, there's not much point in doing this now: bevy_reflect will still be drawn into your tree and other crates will still have reflection information embedded.

[alice-i-cecile]

The initial PR that adds this should not try to feature-gate each subcrate: instead, it should just set up the infrastructure and connect it to any subcrates that already have a reflection feature flag. Feature flagging subcrates should be done one PR per crate.