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/api/testing.go

@@ -129,6 +129,11 @@ func (a *recordingAccessControlFake) RegisterScopeAttributeResolver(prefix strin
 	panic("implement me")
 }
 
+func (a *recordingAccessControlFake) WithoutResolvers() ac.AccessControl {
+	// TODO implement me
+	panic("implement me")
+}
+
 func (a *recordingAccessControlFake) IsDisabled() bool {
 	return a.Disabled
 }