enhancement(nats sink): add support for JetStream message headers

[benjamin-awd]

Summary

This PR adds support in the nats sink for JetStream message headers.

It introduces a configurable, templated Nats-Msg-Id header that ensures a unique ID for each message. This enables broker-level deduplication, resulting in stronger delivery guarantees and exactly-once semantics when combined with idempotent consumers.

This PR also adds proper integration tests for JetStream.

Vector configuration

sinks:
    my_jetstream_sink:
        type: nats
        inputs: [ "my_source" ]
        endpoint: "nats://localhost:4222"
        subject: "my.subject"
        jetstream:  # jetstream: true will still be supported for backwards compatability  
            enabled: true
            headers:
                message_id: "{{ event_id }}"

How did you test this PR?

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: #23509

[pront]

Thanks!

[benjamin-awd]

EDIT: figured it out -- I used two slashes instead of three for the enabled docstring

Running into a weird issue with make generate-component-docs, for some reason the doc-string for enabled isn't going through:

	jetstream: {
	...
		required: false
		type: object: options: {
			enabled: {
				description: "Whether to enable Jetstream."
				required: false
				type: bool: default: false
			}

I guess there must be something I'm missing from my config here:

/// Configuration for sending messages using NATS JetStream.
#[configurable_component]
#[serde_with::serde_as]
#[derive(Clone, Debug, Eq, PartialEq, Default)]
#[serde(deny_unknown_fields)]
pub struct JetStreamConfig {
    // Whether to enable Jetstream.
    #[configurable(derived)]
    #[serde(default)]
    pub enabled: bool,

    /// A map of NATS headers to be included in each message.
    #[configurable(metadata(docs::templateable))]
    #[serde(default)]
    pub(super) headers: Option<NatsHeaderConfig>,
}

impl From<bool> for JetStreamConfig {
    fn from(enabled: bool) -> Self {
        Self {
            enabled,
            ..Default::default()
        }
    }
}