Marshalling a json.RawMessage is not zero overhead. Instead,
it compacts the raw message which starts to have an overhead
at scale.
https://github.com/golang/go/issues/33422
Since we have full control over the message constructed, we
can simply write the byte slice into the network stream.
This gives considerable performance boost.
```
goos: linux
goarch: amd64
pkg: github.com/mattermost/mattermost/server/public/model
cpu: Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz
│ old.txt │ new_2.txt │
│ sec/op │ sec/op vs base │
EncodeJSON-8 1640.5n ± 2% 289.6n ± 1% -82.35% (p=0.000 n=10)
│ old.txt │ new_2.txt │
│ B/op │ B/op vs base │
EncodeJSON-8 528.0 ± 0% 503.0 ± 0% -4.73% (p=0.000 n=10)
│ old.txt │ new_2.txt │
│ allocs/op │ allocs/op vs base │
EncodeJSON-8 5.000 ± 0% 4.000 ± 0% -20.00% (p=0.000 n=10)
```
P.S. No concerns over changing the model API because we are
still using 0.x
https://mattermost.atlassian.net/browse/MM-54998
```release-note
Improve websocket event marshalling performance
```
3563e56a77