Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/fleet/src/main/java/org/elasticsearch/xpack/fleet/Fleet.java
private SystemIndexDescriptor fleetAgentsSystemIndexDescriptor() {
        PutIndexTemplateRequest request = new PutIndexTemplateRequest();
        request.source(loadTemplateSource("/fleet-agents.json"), XContentType.JSON);
        return SystemIndexDescriptor.builder()
            .setType(Type.EXTERNAL_MANAGED)
            .setAllowedElasticProductOrigins(ALLOWED_PRODUCTS)
            .setOrigin(FLEET_ORIGIN)
            .setVersionMetaKey(VERSION_KEY)
            .setMappings(request.mappings())
            .setSettings(request.settings())
            .setPrimaryIndex(".fleet-agents-" + CURRENT_INDEX_VERSION)
            .setIndexPattern(".fleet-agents*")
            .setAliasName(".fleet-agents")
            .setDescription("Configuration of fleet servers")
            .build();
    }
    private SystemIndexDescriptor fleetEnrollmentApiKeysSystemIndexDescriptor() {
        PutIndexTemplateRequest request = new PutIndexTemplateRequest();
        request.source(loadTemplateSource("/fleet-enrollment-api-keys.json"), XContentType.JSON);
        return SystemIndexDescriptor.builder()
            .setType(Type.EXTERNAL_MANAGED)
            .setAllowedElasticProductOrigins(ALLOWED_PRODUCTS)
            .setOrigin(FLEET_ORIGIN)
            .setVersionMetaKey(VERSION_KEY)
            .setMappings(request.mappings())
            .setSettings(request.settings())
            .setPrimaryIndex(".fleet-enrollment-api-keys-" + CURRENT_INDEX_VERSION)
            .setIndexPattern(".fleet-enrollment-api-keys*")
            .setAliasName(".fleet-enrollment-api-keys")
            .setDescription("Fleet API Keys for enrollment")