Support filtering datasets by access_level query parameter - with UI filter change

Currently, Data.gov does not support filtering datasets by the accessLevel metadata field. Users can view the field on individual datasets or retrieve it through the API, but cannot directly search for datasets with a specific access level (for example, non-public).

This limitation was raised by user while trying to identify non-public datasets. ([slack conversation](https://gsa-tts.slack.com/archives/C2N85536E/p1782760050460479))

This feature should be available through the existing search URL/API AND ADD A a new sidebar filter in the Catalog UI.

CHANGED SCOPE OF TICKET _ Marcos will edit if necessary

support value include:
```
public
non-public
restricted public
```

## How to reproduce

`/?access_level=non-public`

## Expected behavior

...would return datasets whose accessLevel is non-public.

## Actual behavior

The query parameter does nothing

## Sketch

- Provides an advanced capability for API users and users who consult the documentation.
- Follows the same pattern that could later be applied to other metadata fields (for example, resource_format).
- Update the Data.gov [User Guide/API documentation](https://data.gov/user-guide/) to describe the new query parameter and provide examples of its usage.