Add out of band loading to rive-wasm

Draft, as there's a quite a few bits to do, but would be neat to get thoughts on it

it "works" but there are questions.

- [x] clean up commits a bit
- [x] pull out cdn stuff to pr that alone & unblock @umberto-sonnino
- [x] make prettier example, i think something with a few images is going to work well
- [x] sort out cpp api for decoding assets. dart lets you set the image, i think cpp needs to do the same. (rather than make it implement decode as it currently does)
- [x] asyncify?! ( we are not doing this at this point)
- [x] make fonts replaceable
- [x] fonts do not replace after rendering/ sometimes async fonts do not get drawn (will tackle this later, need to add artboard referencers to assets so that we know what needs to be made dirty once they are updated)
- [x] fix tests
- [x] add tests
- [x] test webgl
- [x] figure out caching (pushed off for later pr)

example looks a bit like this:

https://github.com/rive-app/rive/assets/1216025/98bc1eaf-6076-44d2-93ca-a9cc8ea42f9e

but it can look like this if the fonts do not load properly

![CleanShot 2023-09-26 at 15 52 56](https://github.com/rive-app/rive/assets/1216025/c8e3f06c-2ef4-4e5a-a780-0f764f4d06b3)

(to repro ☝️ set latency to 300ms)

to run.
compile wasm

`cd rive/packages/runtime_wasm/wasm` && `./build_all_wasm.sh`

run the example
`cd rive/packages/runtime_wasm/wasm/examples/out_of_band_example` && `npm start` (oh and `npm install` first)

Diffs=
f95f54140 Add out of band loading to rive-wasm (#6017)

.rive_head

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