re_format: custom formatters need to be defined and registered from other crates

[teh-cmc]

Registering custom formatters should be done from other crates:

• A) Because re_format cannot depend on other crates (cyclic deps)
• B) Because how to deserialize and inspect some type is a private implementation detail of that type, re_format shouldn't know how to deserialize and format a TUID for example...

The problem is: when to register?
We need those custom formatters to be available at all time: examples, binaries, tests, benchmarks... we can't manually handle the registration for every permutation.

Ideally, we want to do the registration statically, which is tricky.
There are some options though, e.g.:

• https://github.com/dtolnay/inventory
• https://github.com/dtolnay/linkme