MM-11886: materialize channel search (#9349)

* materialize PublicChannels table

Introduce triggers for each supported database that automatically maintain a subset of the Channels table corresponding to only public channels. This improves corresponding queries that no longer need to filter out 99% DM channels.

This initial commit modifies the channel store directly for easier code reviewing, but the next wraps an experimental version around it to enable a kill switch in case there are unforeseen performance regressions.

This addresses [MM-11886](https://mattermost.atlassian.net/browse/MM-11886) and [MM-11945](https://mattermost.atlassian.net/browse/MM-11945).

* extract the experimental public channels materialization

Wrap the original channel store with an experimental version that
leverages the materialized public channels, but can be disabled to
fallback to the original implementation.

This addresses MM-11947.

* s/ExperimentalPublicChannelsMaterialization/EnablePublicChannelsMaterialization/

* simplify error handling

* move experimental config listener until after store is initialized

store/store.go

@@ -174,6 +174,11 @@ type ChannelStore interface {
 	ResetAllChannelSchemes() StoreChannel
 	ClearAllCustomRoleAssignments() StoreChannel
 	ResetLastPostAt() StoreChannel
+	MigratePublicChannels() error
+	DropPublicChannels() error
+	EnableExperimentalPublicChannelsMaterialization()
+	DisableExperimentalPublicChannelsMaterialization()
+	IsExperimentalPublicChannelsMaterializationEnabled() bool
 }
 
 type ChannelMemberHistoryStore interface {