[Fleet] added unprivileged mapping to kql validation (#183239)

## Summary

Fixed error with new `unprivileged` field search when strict KQL
validation is turned on.

elastic/ingest-dev#3250 (comment)

To verify:
- enable KQL validation to `kibana.dev.yml`:
`xpack.fleet.enableExperimental: ['enableStrictKQLValidation']`
- start kibana and enroll an agent with docker
- search on `local_metadata.elastic.agent.unprivileged: true` in Agent
list UI
- expect no error

<img width="1340" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/e00c07eb-0e14-40b5-8ca6-6c559e25224b">

x-pack/plugins/fleet/common/constants/mappings.ts

@@ -166,6 +166,9 @@ export const AGENT_MAPPINGS = {
                     },
                   },
                 },
+                unprivileged: {
+                  type: 'boolean',
+                },
               },
             },
           },

x-pack/plugins/fleet/public/applications/fleet/components/search_bar.test.tsx

@@ -170,7 +170,7 @@ describe('SearchBar', () => {
 
 describe('getFieldSpecs', () => {
   it('returns fieldSpecs for fleet-agents', () => {
-    expect(getFieldSpecs(`.${AGENTS_PREFIX}`)).toHaveLength(66);
+    expect(getFieldSpecs(`.${AGENTS_PREFIX}`)).toHaveLength(67);
   });
   it('returns getFieldSpecs for fleet-enrollment-api-keys', () => {
     const indexPattern = `.${FLEET_ENROLLMENT_API_PREFIX}`;