EVEREST-107 Add workflow for auto-updating PR's base branch (#1016)

This workflow runs on every commit to main and updates the base branch
of every PR with the 'auto-update-base' label.

.github/workflows/auto-update-base.yaml

@@ -0,0 +1,60 @@
+name: Auto-Update PR Base
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  auto-update-base:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/github-script@v7
+        with:
+          script: |
+            try {
+              const { data: pulls } = await github.rest.pulls.list({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                state: "open",
+              });
+
+              for (const pull of pulls) {
+                const hasAutoUpdateLabel = pull.labels.some(label => label.name === "auto-update-base");
+                
+                if (hasAutoUpdateLabel) {
+                  try {
+                    const pr = await github.rest.pulls.get({
+                      owner: context.repo.owner,
+                      repo: context.repo.repo,
+                      pull_number: pull.number,
+                    });
+
+                    if (pr.data.mergeable_state === 'dirty') {
+                      console.log(`[INFO] PR #${pull.number}: "${pull.title}" - Skipping update due to merge conflict.`);
+                      continue;
+                    }
+                    if (pr.data.mergeable_state !== 'behind') {
+                      console.log(`[INFO] PR #${pull.number}: "${pull.title}" - Base branch is up-to-date.`);
+                      continue;
+                    }
+
+                    console.log(`[UPDATE] PR #${pull.number}: "${pull.title}" - Updating base branch...`);
+                    try {
+                      await github.rest.pulls.updateBranch({
+                        owner: context.repo.owner,
+                        repo: context.repo.repo,
+                        pull_number: pull.number,
+                      });
+                      console.log(`[SUCCESS] PR #${pull.number}: "${pull.title}" - Base branch updated.`);
+                    } catch (updateError) {
+                      console.error(`[ERROR] PR #${pull.number}: Failed to update base branch - ${updateError.message}`);
+                    }
+                  } catch (diffError) {
+                    console.error(`[ERROR] PR #${pull.number}: Failed to compare commits - ${diffError.message}`);
+                  }
+                }
+              }
+            } catch (error) {
+              console.error(`[ERROR] Failed to list pull requests - ${error.message}`);
+            }