feat: add custom value support for s3 region (#15968)

### What problem does this PR solve?
Allow S3-compatible data source region fields to accept custom values
while preserving search-and-select behavior.

### Type of change
- [x] New Feature (non-breaking change which adds functionality)

web/src/components/dynamic-form.tsx

@@ -69,6 +69,7 @@ export interface FormFieldConfig {
   required?: boolean;
   placeholder?: string;
   options?: { label: string; value: string }[];
+  allowCustomValue?: boolean;
   defaultValue?: any;
   validation?: {
     pattern?: RegExp;
@@ -456,6 +457,7 @@ export const RenderField = ({
                 triggerClassName="!shrink"
                 {...finalFieldProps}
                 options={field.options}
+                allowCustomValue={field.allowCustomValue}
                 disabled={field.disabled}
               />
             );