[REST API] Add `recursive=true` parameter for `/v1/{prefix}/namespaces` endpoint

### Proposed Change

The `/v1/{prefix}/namespaces` endpoint is currently set up in such a way that it only lists the top-level namespaces, or if `parent` is set, it will list the namespaces directly under that namespace.

If we want to be sure that we have retrieved all namespaces, we need to issue a request for *every* namespace that is returned to be sure that they have no additional nested namespaces.

The idea behind the `recursive=true` parameter is that the server does this traversal in a single request, reducing round trip times.
This shouldn't be an issue, because the `Namespace` object that is returned already contains the full list of `items` that make up the full path of the namespace.

An alternative (but worse) solution, to make sure we don't need to make `parent=` requests for *every* namespace just to verify that there are no additional namespaces, is to indicate in the `ListNamespacesResponse` or `Namespace` object that a namespace is nested.
That way the client does not need to make requests that end up in an empty result.

### Proposal document

_No response_

### Specifications

- [ ] Table
- [ ] View
- [x] REST
- [ ] Puffin
- [ ] Encryption
- [ ] Other

Start by locating the REST implementation for /v1/{prefix}/namespaces and review how parent filtering currently works. Then inspect the Namespace and ListNamespacesResponse shapes; done means recursive=true returns namespaces at all nesting levels in one request while existing behavior remains available.

Written by the indexing model from the issue text.