Allow flex time to be filled by more than just filler content

[nappillow]

I feel like there are many ways to build content sets in Tunarr, but I can't use them all interchangeably. Here's an example:

I'd like to just have a channel of Dropout/CollegeHumor shorts running 24/7, and for channels where I deem the shorts appropriate for breaks, just cut over to them. Mid play, whatever. I don't think I care about right sizing them, just whatever happens to be on there at any given time. Maybe that model is more suited to a music channel than a bunch of clips, but nevertheless, it feels like there's a place for it. I'd like to accomplish this with a channel redirect.

In researching how I might apply this, I looked at both expanding the scope of how "filler" content is defined and at expanding options for Flex time, and I feel like allowing Flex time to point at different things a la:

FlexProgrammingSlotSchema = z.object({
  type: z.literal('flex'),
  fillSource: z.discriminatedUnion('type', [
    z.object({type: z.literal('dead-air')}),         // current behavior
    z.object({type: z.literal('filler-list'), fillerListId, ...}),
    z.object({type: z.literal('channel-redirect'), channelId}),
    z.object({type: z.literal('custom-show'), customShowId, ordering}),
    z.object({type: z.literal('smart-collection'), smartCollectionId, ordering}),
  ]).default({type: 'dead-air'}),
})

feels more like the right shape to me.

Discussion #1399 raised the same shape for music playlists; this extends it to channel-as-filler more generally.