Remove valyala/fastjson and valyala/fasttemplate dependencies (#37914)
<!-- Add the related story/sub-task/bug number, like Resolves #123, or
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Context:
https://fleetdm.slack.com/archives/C019WG4GH0A/p1767713469571139
Replaced `valyala` dependencies and now relying on `json.Unmarshal` and
manual traversal of `Template` subjects, such as [this
one](https://github.com/fleetdm/fleet/blob/main/server/logging/nats_test.go#L113)
# Checklist for submitter
## Testing
- [x] ~~Added/updated automated tests~~ I'm relying on existing tests on
`nats_test.go` which already cover using a `Template` subject, namely:
https://github.com/fleetdm/fleet/blob/main/server/logging/nats_test.go#L112-L132
https://github.com/fleetdm/fleet/blob/main/server/logging/nats_test.go#L194-L245
https://github.com/fleetdm/fleet/blob/main/server/logging/nats_test.go#L301-L356
- [ ] Where appropriate, [automated tests simulate multiple hosts and
test for host
isolation](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/reference/patterns-backend.md#unit-testing)
(updates to one hosts's records do not affect another)
- [x] QA'd all new/changed functionality manually
Ran `nats-server`, subscribed to all subjects by running `nats
--server=nats://localhost:4222 subscribe ">"` and got logs from this
query:
<img width="675" height="411" alt="Screenshot 2026-01-06 at 4 12 52 PM"
src="https://github.com/user-attachments/assets/e4e6e5d0-53ac-4b09-9810-b6032794d5f3"
/>
<img width="773" height="165" alt="Screenshot 2026-01-06 at 4 11 16 PM"
src="https://github.com/user-attachments/assets/6f58d1f1-272b-40b3-96f5-1659c0bbb918"
/>
<img width="2541" height="119" alt="Screenshot 2026-01-06 at 4 11 06 PM"
src="https://github.com/user-attachments/assets/2e61acac-063c-4cdd-aeee-871031600125"
/>
Nico
116c8ddb4f