Change the UI to support agent registration with and without a config

What does this MR do and why?

This MR adds a possibility to register an agent without a config file.

Changes within the MR:

1. Install agent modal
   • the empty state is shown only when KAS is disabled
   • when KAS is enabled, the dropdown to select or create a config is shown
   • copy changed for the KAS enabled state of the modal
2. Agent selection dropdown:
   • the search field is added using the gl-search-by-type component
   • the button to create an agent if nothing is found added
   • an error is returned from KAS if the agent name is not supported
3. Agent table:
   • the Default config is shown for the agents without a config file
   • the question mark is shown that leads to the documentation

Screenshots or screen recordings

Adding agent without a config

Install agent modal when KAS is disabled

Install agent modal when KAS is enabled

Error returned for the incorrect agent name

Agent with the default config in agents list

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit Infrastructure -> Kubernetes Clusters
2. Select Actions to open Install agent modal
3. Use agent selection dropdown to search for the existing agent config or create an agent without a config
4. Verify the agent details in the agent table after successful creation.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #347240 (closed)