Add default dependabot configurations

The intention with these is that crates including rust-vmm-ci as a
submodule will be able to copy either the monthly or weekly schedule to
their repository's .github/dependabot.yml. Sadly, it is not possible to
automatically keep these in-sync with upstream, as dependabot does not
support symlinking the dependabot.yml file into a submodule. However, we
expect updates to be rare enough for this to not be a practical problem.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

dependabot-monthly.yml

@@ -0,0 +1,25 @@
+version: 2
+updates:
+
+# A monthly update of the rust-vmm-ci submodule
+- package-ecosystem: gitsubmodule
+  directory: "/"
+  schedule:
+    interval: monthly
+  open-pull-requests-limit: 1
+
+# A monthly update to rust dependencies. These will be grouped,
+# e.g. one PR will contains updates for all dependencies.
+- package-ecosystem: cargo
+  directory: "/"
+  schedule:
+    interval: monthly
+  open-pull-requests-limit: 1
+  # Make it also update transitive dependencies in Cargo.lock
+  allow:
+    - dependency-type: "all"
+  # Group all available updates into a group called "rust-dependencies"
+  groups:
+    rust-dependencies:
+      patterns:
+        - "*"

dependabot-weekly.yml

@@ -0,0 +1,27 @@
+version: 2
+updates:
+
+# A weekly update of the rust-vmm-ci submodule
+- package-ecosystem: gitsubmodule
+  directory: "/"
+  schedule:
+    interval: weekly
+    day: monday
+  open-pull-requests-limit: 1
+
+# A weekly update to rust dependencies. These will be grouped,
+# e.g. one PR will contains updates for all dependencies.
+- package-ecosystem: cargo
+  directory: "/"
+  schedule:
+    interval: weekly
+    day: monday
+  open-pull-requests-limit: 1
+  # Make it also update transitive dependencies in Cargo.lock
+  allow:
+    - dependency-type: "all"
+  # Group all available updates into a group called "rust-dependencies"
+  groups:
+    rust-dependencies:
+      patterns:
+        - "*"