[Search Connectors][Serverless] Add Search Connectors UI to the Stack Management data Section (#213509)

**!!MAJORITY OF THE CHANGED FILES ARE MOVED OR COPIED!!**

### Vision

According to the product vision we will build a new simple UI/UX in the
future https://github.com/elastic/security-team/issues/11790

This PR is a first iteration on enabling Content Connectors Management
UI in Serverless Kibana Stack Management.
Elastic Managed content connectors will be available only for Security
and Observability projects.
### Current PR scope

1. Used initial search_connectors plugin and renamed it to
content_connectors + moved from `x-pack/solutions/search` to
`x-pack/platform/plugins/shared`
2. Copy relevant connectors UI and routes from enterprise_search plugin.
3. Introduce the new Stack Management card/navigation option under the
Data section.
4. Enabled this plugin only in Serverless for Security and Observability
projects.
5. For making PR smaller Pipelines tab was not moved. And according to
Search team vision this functionality should be dropped anyway soon.
6. Extended fleet package logic to include elastic_connectors for
security and o11y serverless projects
7. Added back `search:agentless-connectors-manager` task

In Stack Management navigation:
<img width="2062" alt="Screenshot 2025-04-15 at 3 51 43 PM"
src="https://github.com/user-attachments/assets/5c93ba01-9a6a-4eac-a21d-1370f03b8f35"
/>

Stack Management cards:
<img width="2081" alt="Screenshot 2025-04-10 at 8 41 43 PM"
src="https://github.com/user-attachments/assets/3def1c12-561b-4a84-8241-4dd61cd9313d"
/>


Create Elastic Managed Connector UI (on Agentless):

<img width="1822" alt="Screenshot 2025-04-15 at 3 55 29 PM"
src="https://github.com/user-attachments/assets/6e9fea48-85e7-43df-919d-0e5492d0e704"
/>

Create Self Managed Connector UI:

<img width="2064" alt="Screenshot 2025-04-15 at 3 55 49 PM"
src="https://github.com/user-attachments/assets/d5051898-c8fa-4e41-b9ea-b41d4ed4a0d5"
/>

### Next steps

- [ ] Remove duplicated code between content_connectors,
enterprise_search and serverless_search
- [ ] Extract [common server
libs](https://github.com/elastic/kibana/tree/main/x-pack/solutions/search/plugins/enterprise_search/server/lib)
to the shared package `kbn-search-connectors`

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Steph Milovic <stephanie.milovic@elastic.co>
Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
Co-authored-by: Artem Shelkovnikov <artem.shelkovnikov@elastic.co>
Co-authored-by: Artem Shelkovnikov <lavatroublebubble@gmail.com>
Co-authored-by: Kyle Pollich <kyle.pollich@elastic.co>

packages/kbn-optimizer/limits.yml

@@ -18,6 +18,7 @@ pageLoadAssetSize:
   cloudLinks: 55984
   cloudSecurityPosture: 34398
   console: 61298
+  contentConnectors: 65000
   contentManagement: 16254
   controls: 12000
   core: 564663
@@ -130,7 +131,6 @@ pageLoadAssetSize:
   screenshotMode: 17856
   screenshotting: 22870
   searchAssistant: 19831
-  searchConnectors: 65000
   searchHomepage: 19831
   searchIndices: 20519
   searchInferenceEndpoints: 20470