oc-mirror requires that the default channel of an operator is mirrored

* Prior to this release, to filter operator packages by channel, for example, `mirror.operators.catalog.packages.channels`, you had to specify the default channel for the package, even if you did not intend to use the packages from that channel. Based on this information, the resulting catalog is considered invalid if the `imageSetConfig` does not contain the default channel for the package. This update introduces the `defaultChannel` field in the `mirror.operators.catalog.packages` section. You can now select a default channel. This action enables `oc-mirror` to build a new catalog that defines the selected channel in the `defaultChannel` field as the default for the package. [ OCPBUGS-385 | https://issues.redhat.com/browse/OCPBUGS-385 ]