feat: add withLabels function to utils for enhanced rule labeling

[andriikushch]

This PR adds a new utility function withLabels() to the mixin-utils library that allows adding labels to alert rules.

Features

• Add labels to all alerts in rule groups
• Optionally filter which alerts receive the labels using a custom filter function
• Preserves existing labels on alerts

Usage example:

local utils = import 'mixin-utils/utils.libsonnet';

// Add environment and team
local alertGroups = [...];
local withEnvironment = utils.withLabels(
  { environment: 'production', team: 'myteam' },
  alertGroups
);

[tristanburgess]

nice work!