Add loki.enrich component (WIP)

[v-zhuravlev]

PR Description

This PR adds a new loki.enrich component that enriches logs with labels from discovery.* components.
The component matches a label from incoming logs against a label from discovered targets, and copies specified labels from the matched target to the log entry.

Features:

• Match logs to targets using configurable labels
• Copy selected labels from matched targets to logs
• Support for any discovery component (file, DNS, HTTP, etc.)
• Caching of target labels
• Thread-safe label updates

Example use cases:

• Enrich Network devices syslog messages with device metadata from Netbox (https://github.com/FlxPeters/netbox-prometheus-sd)
• Enrich logs received through loki.source.api with additional labels in order to easily correlate logs and metrics.

Notes to the Reviewer

The enrich component:

1. Uses a map to cache target labels for efficient lookups
2. Updates cache atomically when targets change
3. Forwards logs unchanged if no match is found

The integration test demonstrates:

• Log pushed via HTTP API
• Label enrichment using file-based discovery sample (could be some external source instead)

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[github-actions]

💻 Deploy preview available:

• https://deploy-preview-alloy-2882-zb444pucvq-vp.a.run.app/docs/alloy/latest/