* Use kubebuilder client for fetching restic secrets

Instead of using a SecretInformer for fetching secrets for restic, use
the cached client provided by the controller-runtime manager.

In order to use this client, the scheme for Secrets must be added to the
scheme used by the manager so this is added when creating the manager in
both the velero and restic servers.

This change also refactors some of the tests to add a shared utility for
creating a fake controller-runtime client which is now used among all
tests which use that client. This has been added to ensure that all
tests use the same client with the same scheme.
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>

* Add builder for SecretKeySelector
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>