Update the `ClusterExtension.Spec.Channel` field to be a list of channels

[everettraven]

Based on discussions for v1.0.0 features, we are heading into a reality where a version range can be specified and a set of catalogs can be specified. We should allow for a set of channels to be specified as well. This would mean:

• ClusterExtension.Spec.Channel would become ClusterExtension.Spec.Channels
• The field would remain optional
• The field would now be a list of channel names
• If the channel does not exist, no bundles will be returned for that channel (i.e nothing happens). If none of the channels exist, no bundle would be resolved and a resolution error would occur

[everettraven]

Not marking this one as a good first issue as there are likely resolution changes involved. I'm not sure the realm of resolution is great for a first time contributor in this case.