api: expose server search configuration

### Proposal

`GET /v1/search/config` (?) could respond with

```
{
"FuzzyEnabled": false,
…
}
```

or

```
{
"MinTermLength": 5,
…
}
```

### Use-cases

The fuzzy search UX via the web UI could be improved if the application knew:
• whether fuzzy search was disabled
• what the minimum search term length is before results will be returned

The UI currently runs a [feature detection fuzzy search query](https://github.com/hashicorp/nomad/blob/92391683da29a6f11c2b375c0b9880a27acea8fa/ui/app/services/system.js#L136-L151) on application load and hides the search control when the `500` response indicates `fuzzy search is not enabled`. It would be nice to be able to learn this directly vs via a fake query.

The UI’s search is [currently hard-coded](https://github.com/hashicorp/nomad/blob/92391683da29a6f11c2b375c0b9880a27acea8fa/ui/app/components/global-search/control.js#L133-L136) to require a minimum of two characters before dispatching a query. If the fuzzy search is configured with `min_term_length: 5` or anything greater than 2, the search shows “No results found” for a shorter because there’s no way to know what the configured value is. (It appears to be expected that the endpoint returns [an error](https://github.com/hashicorp/nomad/blob/0af476263b5c6f12a43a069070400e1762ad17c2/nomad/search_endpoint_test.go#L1112) that the configuration could be derived from, but that’s not working for me, I’ll document that separately.)

Start with nomad/search_endpoint_test.go and the referenced UI code in ui/app/services/system.js and ui/app/components/global-search/control.js. Review how fuzzy-search availability and minimum term length are currently detected or hard-coded. Done means the API exposes the relevant search configuration so the UI no longer needs a fake query or fixed threshold.

Written by the indexing model from the issue text.