RBAC: Add required component to perform access control checks for user api when running single tenant (#93104)

* Unexport store and create new constructor function

* Add ResourceAuthorizer and LegacyAccessClient

* Configure checks for user store

* List with checks if AccessClient is configured

* Allow system user service account to read all users

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Co-authored-by: Gabriel MABILLE <gamab@users.noreply.github.com>

pkg/services/ngalert/accesscontrol/testing.go

@@ -32,8 +32,8 @@ func (a *recordingAccessControlFake) RegisterScopeAttributeResolver(prefix strin
 	panic("implement me")
 }
 
-func (a *recordingAccessControlFake) IsDisabled() bool {
-	return a.Disabled
+func (a *recordingAccessControlFake) WithoutResolvers() accesscontrol.AccessControl {
+	panic("unimplemented")
 }
 
 var _ accesscontrol.AccessControl = &recordingAccessControlFake{}