Add dependabot.yml for dependency version updates (#1296)

### What

Adding base config for `dependabot.yml`, which enables automatic PRs for
[dependency version
updates](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates)
through Dependabot. This base config sets version updates to run on a
`weekly` schedule, groups PRs for version updates according to `major`
vs `minor/patch` updates, and sets max PRs that Dependabot can create
for version updates per package-ecosystem.
But if you would like, several config options can be added to
`dependabot.yml` as per
[configuration-options-for-the-dependabot.yml-file](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file)
to customize version updates, such as:

- Group PRs for version updates by [different
criteria](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups)
- Set `dependency-type` option in [allow
option](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#allow)
to `all` for enabling version updates for both direct and indirect
dependencies (as well as prod and dev dependencies). By default, PRs for
only updating direct prod+dev dependencies versions are created.
- Set
[reviewers](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#reviewers)
for Dependabot version updates PRs
- Etc.

Also, automatic PRs for [security
updates](https://docs.github.com/en/code-security/dependabot/dependabot-security-updates/about-dependabot-security-updates)
through Dependabot are enabled through a global setting. PRs for
security updates will be grouped as much as possible across directories
and per ecosystem through the global setting
([grouping-dependabot-security-updates](https://docs.github.com/en/code-security/dependabot/dependabot-security-updates/configuring-dependabot-security-updates#grouping-dependabot-security-updates-into-a-single-pull-request)).
But `dependabot.yml` can also be used for more granular security updates
settings in addition to version updates (some of the options in
`dependabot.yml` are shared across security and version updates), such
as:

- Grouping security updates according to custom criteria:
[overriding-the-default-behavior-with-a-configuration-file](https://docs.github.com/en/code-security/dependabot/dependabot-security-updates/configuring-dependabot-security-updates#overriding-the-default-behavior-with-a-configuration-file)
- Only allowing automatic PRs for security updates for certain
dependencies by configuring [allow
directive](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#allow)
to a certain value such as `production, development, direct, indirect`.
By default, security updates are applied to all dependencies (direct,
indirect, production, development).
- Etc.

### Why

To enable using the latest package versions, and provide a way to
customize Dependabot security updates PRs.

### Known limitations

N/A

---------

Co-authored-by: Leigh McCulloch <351529+leighmcculloch@users.noreply.github.com>

Author: kanwalpreetd

.github/dependabot.yml

@@ -0,0 +1,36 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directories:
+      - "/"
+      - "/soroban-ledger-snapshot"
+      - "/soroban-sdk-macros"
+      - "/soroban-sdk"
+      - "/soroban-spec-rust"
+      - "/soroban-spec"
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+      time: "02:00"
+    open-pull-requests-limit: 2
+    groups:
+      minor-and-patch:
+        applies-to: version-updates
+        update-types:
+        - "patch"
+        - "minor"
+      major:
+        applies-to: version-updates
+        update-types:
+        - "major"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+      time: "02:00"
+    open-pull-requests-limit: 2
+    groups:
+      all-actions:
+        applies-to: version-updates
+        patterns: [ "*" ]