MM-8678: add CUD support for channel members via plugins (#8565)

* add CUD support for channel members via plugins

This effectively exposes AddChannelMember, UpdateChannelMemberRoles,
UpdateChannelMemberNotifyProps and LeaveChannel via the plugin API.

It also modifies the semantics of AddChannelMember to explicitly allow
for an empty user requestor, left as such for now via the plugin API.

* change the signature of AddChannelMember to accept a channel id instead of a channel

plugin/api.go

@@ -77,9 +77,21 @@ type API interface {
 	// UpdateChannel updates a channel.
 	UpdateChannel(channel *model.Channel) (*model.Channel, *model.AppError)
 
+	// AddChannelMember creates a channel membership for a user.
+	AddChannelMember(channelId, userId string) (*model.ChannelMember, *model.AppError)
+
 	// GetChannelMember gets a channel membership for a user.
 	GetChannelMember(channelId, userId string) (*model.ChannelMember, *model.AppError)
 
+	// UpdateChannelMemberRoles updates a user's roles for a channel.
+	UpdateChannelMemberRoles(channelId, userId, newRoles string) (*model.ChannelMember, *model.AppError)
+
+	// UpdateChannelMemberNotifications updates a user's notification properties for a channel.
+	UpdateChannelMemberNotifications(channelId, userId string, notifications map[string]string) (*model.ChannelMember, *model.AppError)
+
+	// DeleteChannelMember deletes a channel membership for a user.
+	DeleteChannelMember(channelId, userId string) *model.AppError
+
 	// CreatePost creates a post.
 	CreatePost(post *model.Post) (*model.Post, *model.AppError)