chore: Add "Willingness to contribute" dropdown to issue templates (#26120)

## Which issue does this PR close?

- No tracking issue, this is a small repository chore.

## Rationale for this change

Ports apache/datafusion-comet#6688, which took
the idea from the [iceberg-rust issue
templates](https://github.com/apache/iceberg-rust/tree/main/.github/ISSUE_TEMPLATE).
Reporters say up front whether they can contribute the fix or feature,
so reviewers don't have to ask in a follow-up comment.

## What changes are included in this PR?

Adds an optional "Willingness to contribute" dropdown with three options
to `bug_report.yml` and `feature_request.yml`. The wording is specific
to bug fixes in the bug template and to features in the feature
template.

## What is the testing strategy for this PR?

Issue templates have no automated tests. Both files parse as YAML and
the dropdown has the same shape as in the iceberg-rust and Comet
templates.

## Are there any user-facing changes?

New issues filed from the bug report and feature request forms get one
extra optional dropdown. There are no API changes.

Author: comphead

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -26,3 +26,12 @@ body:
       label: Additional context
       placeholder: >
         Add any other context about the problem here.
+  - type: dropdown
+    attributes:
+      label: Willingness to contribute
+      description: >
+        The Apache DataFusion community encourages bug fix contributions. Would you or another member of your organization be willing to contribute a fix for this bug to the Apache DataFusion codebase?
+      options:
+        - I plan to contribute a fix for this bug independently
+        - I would like to contribute a fix for this bug, but request guidance from the DataFusion community
+        - I cannot contribute a fix for this bug at this time

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -25,3 +25,12 @@ body:
       label: Additional context
       placeholder: >
         Add any other context or screenshots about the feature request here.
+  - type: dropdown
+    attributes:
+      label: Willingness to contribute
+      description: >
+        The Apache DataFusion community encourages feature contributions. Would you or another member of your organization be willing to contribute to this feature for the Apache DataFusion codebase?
+      options:
+        - I plan to contribute to this feature independently
+        - I would like to contribute to this feature, but request guidance from the DataFusion community
+        - I cannot contribute to this feature at this time