Support Idempotent Pipeline Creation/Updates with az pipelines update --create

**Is your feature request related to a problem? Please describe.**

Currently I have a Makefile with a handful of `az pipelines` targets in it. They look something like this:

```bash
PIPELINE_NAME="Prod Environment - Deploy Platform Resources"

if az pipelines show --name "$PIPELINE_NAME" >/dev/null 2>&1; then
echo "Pipeline exists. Updating..."
az pipelines update \
--name "$PIPELINE_NAME" \
--repository PlatformInfrastructure \
--branch main \
--yml-path .azuredevops/pipelines-v2/deploy-platform-resources-prod.yaml \
--repository-type tfsgit \
--folder-path "\Platform\Infrastructure" \
--skip-first-run true
else
echo "Pipeline doesn't exist. Creating..."
az pipelines create \
--name "$PIPELINE_NAME" \
--repository PlatformInfrastructure \
--branch main \
--yml-path .azuredevops/pipelines-v2/deploy-platform-resources-prod.yaml \
--repository-type tfsgit \
--folder-path "\Platform\Infrastructure" \
--skip-first-run true
fi
```

This repetitive conditional logic makes automation more verbose than it needs to be. Other CLI tools like `helm`, `kubectl`, and `terraform` support more declarative workflows that just work regardless of the current state.

---

**Describe the solution you'd like**

There are two potential approaches that would improve the developer experience here:

---

**Option 1: Add a new command like `az pipelines apply`**

Introduce a new command similar to `kubectl apply` or `helm upgrade --install`:

```bash
az pipelines apply \
--name "Prod Environment - Deploy Platform Resources" \
--repository PlatformInfrastructure \
--branch main \
--yml-path .azuredevops/pipelines-v2/deploy-platform-resources-prod.yaml \
--repository-type tfsgit \
--folder-path "\Platform\Infrastructure" \
--skip-first-run true
```

This would:
- Create the pipeline if it doesn’t exist
- Update it if it does

---

**Option 2: Extend `az pipelines update` with a `--create` flag**

Allow `az pipelines update` to create the pipeline if it doesn’t exist by adding a `--create` flag:

```bash
az pipelines update \
--name "Prod Environment - Deploy Platform Resources" \
--repository PlatformInfrastructure \
--branch main \
--yml-path .azuredevops/pipelines-v2/deploy-platform-resources-prod.yaml \
--repository-type tfsgit \
--folder-path "\Platform\Infrastructure" \
--skip-first-run true \
--create
```

This would preserve compatibility with existing commands and avoid introducing a new top-level verb.

---

**Additional context**

This would significantly simplify automation in scripts and Makefiles. Supporting idempotent pipeline declarations aligns with best practices in modern infrastructure tooling and improves UX for GitOps-style workflows.