feat(nats): Added feature flag for websockets in async-nats

[gedemagt]

Summary

This PR adds websockets to async-nats.

Vector configuration

sources:
  nats-input:
    type: nats
    subject: input
    url: ws://localhost:8080
    connection_name: test

sinks:
  nats:
    type: nats
    inputs:
      - nats-input
    subject: test
    url: ws://localhost:8080
    encoding:
      codec: json

How did you test this PR?

Running a nats server with the following docker-compose.yml

configs:
  nats-server.conf:
    content: |
      server_name: sitehub
      websocket {
          port: 8080
          no_tls: true
      }

services:
  nats:
    image: nats:2.11.8
    ports:
      - "8080:8080"
      - "4222:4222"
    configs:
      - nats-server.conf
    command:
      - "--config=/nats-server.conf"

The Vector config specified in the previous section was used, and then the data feed through was tested with a few nats commands:

nats sub test

nats pub input Hello

This results in the following on the subscription

20:25:04 Subscribing on test
[#1] Received on "test"
{"message":"Hello","source_type":"nats","subject":"input","timestamp":"2025-11-21T19:25:33.011930438Z"}

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Closes: NATS does not support Websocket transport. #24285

Notes

• Please read our Vector contributor resources.
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[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[gedemagt]

I have read the CLA Document and I hereby sign the CLA

[thomasqueirozb]

recheck

[gedemagt]

Do I need to do more work on this to get a review?

[thomasqueirozb]

Hi @gedemagt, can you add a changelog? This is a nice addition.

Also, do you know what the behavior was previously? Did it panic/crash, fail with some (useful) error message or silently fail?