update agent policy schema to include overrides functionality

You can use this setting to override custom settings which aren't available in fleet UI.
For eg. https://www.elastic.co/guide/en/fleet/current/enable-custom-policy-settings.html and https://support.elastic.dev/knowledge/view/06b69893

This will make things much easier for elastic/elastic-agent#5648

Author: VihasMakwana

kibana/fleet.go

@@ -78,6 +78,7 @@ type AgentPolicy struct {
 	UnenrollTimeout    int                       `json:"unenroll_timeout,omitempty"`
 	InactivityTImeout  int                       `json:"inactivity_timeout,omitempty"`
 	AgentFeatures      []map[string]interface{}  `json:"agent_features,omitempty"`
+	Overrides          []map[string]interface{}  `json:"overrides,omitempty"`
 	IsProtected        bool                      `json:"is_protected"`
 }
 
@@ -110,6 +111,7 @@ type AgentPolicyUpdateRequest struct {
 	UnenrollTimeout    int                       `json:"unenroll_timeout,omitempty"`
 	InactivityTImeout  int                       `json:"inactivity_timeout,omitempty"`
 	AgentFeatures      []map[string]interface{}  `json:"agent_features,omitempty"`
+	Overrides          []map[string]interface{}  `json:"overrides,omitempty"`
 	IsProtected        *bool                     `json:"is_protected,omitempty"` // Optional bool for compatibility with the older pre 8.9.0 stack
 }