Add VMR PoC CI yaml (#17905)

eng/pipelines/vmr-vertical-build-poc.yml

@@ -0,0 +1,52 @@
+trigger: none
+pr:
+  branches:
+    include:
+    - main
+
+parameters:
+- name: vmrBranch
+  displayName: dotnet/dotnet branch to push to
+  type: string
+  default: ' '
+
+- name: disableVmrBuild
+  displayName: Skip source-building the VMR
+  type: boolean
+  default: false
+
+variables:
+- ${{ if ne(parameters.vmrBranch, ' ') }}:
+  - name: VmrBranch
+    value: ${{ replace(parameters.vmrBranch, ' ', '') }}
+- ${{ else }}:
+  - name: VmrBranch
+    value: ${{ replace(replace(variables['System.PullRequest.TargetBranch'], 'refs/heads/', ''), 'refs/pull/', '') }}
+
+resources:
+  repositories:
+  - repository: vmr
+    type: github
+    name: dotnet/dotnet
+    endpoint: dotnet
+    ref: ${{ variables.VmrBranch }}
+
+stages:
+- ${{ if not(parameters.disableVmrBuild) }}:
+  - template: templates/stages/vmr-cross-build.yml
+    parameters:
+      vmrBranch: ${{ variables.VmrBranch }}
+      isBuiltFromVmr: false
+      isLiteBuild: true
+
+# In case the VMR Build stage is temporarily disabled, the VMR synchronization step is run to validate
+# that the PR can be merged and later synchronized into the VMR without problems.
+- ${{ else }}:
+  - stage: Synchronize_VMR
+    displayName: Synchronize VMR
+    dependsOn: []
+    jobs:
+    - template: templates/jobs/vmr-synchronization.yml
+      parameters:
+        vmrBranch: ${{ variables.VmrBranch }}
+        noPush: true