Add Dependabot config for .NET, NuGet, and GitHub Actions

Introduced a Dependabot configuration to automate dependency
updates for `.NET SDK`, `NuGet` packages, and `GitHub Actions`.

- Added `version: 2` to specify the configuration version.
- Configured `.NET SDK` updates to run weekly on Tuesdays
  with a limit of 1 open pull request.
- Configured `NuGet` updates to run weekly on Sundays with
  a limit of 3 open pull requests, grouping dependencies
  by patterns (e.g., `microsoft-sbom`, `testcontainers`, etc.).
- Configured `GitHub Actions` updates to run weekly on Sundays
  with a limit of 3 open pull requests.
- Disabled the `rebase-strategy` for `NuGet` and `GitHub Actions`.

Signed-off-by: Santhosh Reddy Vootukuri (SUNNY) <nagavo@microsoft.com>

Author: smartysanthosh

.github/dependabot.yml

@@ -0,0 +1,40 @@
+version: 2
+updates:
+  #  https://devblogs.microsoft.com/dotnet/using-dependabot-to-manage-dotnet-sdk-updates/
+  - package-ecosystem: "dotnet-sdk"
+    directory: /
+    schedule:
+      interval: "weekly"
+      day: "tuesday"
+    open-pull-requests-limit: 1
+
+  # NuGet package updates
+  - package-ecosystem: nuget
+    directory: /
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+    open-pull-requests-limit: 3
+    rebase-strategy: disabled
+    groups:
+      microsoft-sbom:
+        patterns: ['Microsoft.Sbom.Targets']
+      Microsoft.NET.Test.Sdk:
+        patterns: ['Microsoft.NET.Test.Sdk']
+      testcontainers:
+        patterns: [ 'Testcontainers', 'Testcontainers.*']
+      microsoft:
+        patterns: [Microsoft.*, System.*]
+      xunit:
+        patterns: [xunit.*]
+      all-dependencies:
+        patterns: ['*']
+
+# Github Actions updates
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+    open-pull-requests-limit: 3
+    rebase-strategy: disabled