Provide feature flags for enabling/disabling controllers

[jhrozek]

Depends on #2556

The current version of toolhive operator is a monolith that consists of many controllers for quite a few CRDs. To support more targeted deployments, we should provide feature flags that let a user enable or disable controllers.

We proposed the following split to enable the CRDs and controller by the logical functions:

FLAG	Affected CRDs	Affected controllers	Dependencies
ENABLE_SERVER	MCPServer, MCPExternalAuthConfig	MCPRemoteProxyReconciler, ToolConfigReconciler	MCPServerReconciler, MCPExternalAuthConfigReconciler, MCPRemoteProxyReconciler, ToolConfigReconciler
ENABLE_REGISTRY	MCPRegistry	MCPRegistryReconciler
ENABLE_AGGREGATION	VirtualMCPServer, MCPGroup	VirtualMCPServerReconciler, MCPGroupReconciler	ENABLE_SERVER

E.g., to enable server management the ENABLE_SERVER must be set to true.