Feature Request: Add directiveIsRepeatable option to schema-ast plugin

### Is your feature request related to a problem? Please describe.

I'm currently working with GraphQL schemas that use repeatable directives (like Apollo Federation's `@key` directive), and when I generate SDL files using the `schema-ast` plugin, the `repeatable` keyword is missing from the output.

This seems to happen because:

1. When loading a schema from a remote URL, the CLI uses an introspection query that doesn't include the `isRepeatable` field
2. The `schema-ast` plugin then calls `printSchema()` on the loaded schema
3. Since the `isRepeatable` information wasn't included in the introspection, the `repeatable` keyword cannot be included in the generated SDL

I found that the `introspection` plugin already has a `directiveIsRepeatable` option (added in [#4810]()), which works well for generating JSON introspection files. It would be great if the `schema-ast` plugin could have similar functionality for SDL generation.

### Describe the solution you'd like

Could you add a `directiveIsRepeatable` option to the `schema-ast` plugin configuration? This would allow users to include the `repeatable` keyword in generated SDL files:

```yaml
generates:
./schema.graphql:
plugins:
- schema-ast
config:
includeDirectives: true
includeIntrospectionTypes: true
directiveIsRepeatable: true # Include repeatable keyword for directives
```

This would ensure that repeatable directives are properly marked with the `repeatable` keyword in the generated SDL, maintaining consistency with the original schema definition.

### Describe alternatives you've considered

1. Custom schema loader: Works but requires boilerplate code
2. Two-step process: Download introspection manually, then use file - breaks the streamlined workflow
3. Using introspection plugin: Only generates JSON, doesn't help with schema-ast plugin

### Any additional important details?

This would make the schema loading behavior consistent with the introspection plugin options and allow full feature parity when working with modern GraphQL schemas that use repeatable directives (like Apollo Federation).
Related:

[#4731]() - Added introspection options to introspection plugin
[#4810]() - PR that implemented the above

The same getIntrospectionQuery options should be available for schema loading as they are for the introspection plugin.