Azure.Provisioning: Add BicepFunction.Union()

[m-nash]

Problem

BicepFunction in Azure.Provisioning does not expose Bicep's union() built-in function. This is needed for non-destructive updates to existing resources — most commonly, adding a tag to an existing resource group without replacing all existing tags.

Motivating scenario

Adding a tag to an existing resource group without removing existing tags requires union():

resource existingRg 'Microsoft.Resources/resourceGroups@2023-07-01' existing = {
  name: 'my-demo-rg'
}

resource updatedRg 'Microsoft.Resources/resourceGroups@2023-07-01' = {
  name: 'my-demo-rg'
  location: existingRg.location
  tags: union(existingRg.tags, { env: 'prod' })  // merge, don't replace
}

This cannot be expressed today through the Azure.Provisioning API. Users must either accept destructive tag replacement or drop down to raw Bicep.

Proposed API

public static BicepValue<object> Union(params BicepValue<object>[] values);

// Usage
updatedRg.Tags = BicepFunction.Union(existingRg.Tags, new BicepDictionary<string> { ["env"] = "prod" });

[m-nash]

Closing — the motivating scenario (non-destructive tag merge) isn't actually solvable with union() alone due to ARM PUT semantics. Will refile if a valid use case emerges.