[Fleet] Remove / disable grouping of ephemeral agents

[mattapperson]

For now we are pulling this concept out of phase 1. Listing of agents should by default NOT list agents that are ephemeral and not seen after 3x the default retry time. but should have a flag in the API to include them. And a toggle on the UI for this.

// cc @mdefazio on UI tweak
// cc @roncohen on existence of issue

[mdefazio]

I see this as a toggle in the Agent table's action bar. We have a placeholder for a toggle in the current prototype, but we haven't decided upon what these will be. I will update the prototype to make this Show Inactive (off by default). But are there other toggles needed here?

Do we need an expand/collapse table row that shows all the grouped agents? Or is it enough to simply know that there are X number of these?