[ui] Jobspec UI block: Descriptions and Links

[philrenaud]

Adds a ui block to Nomad job specs. Users can define any number of links and a single description that will show up on the Job index page in the web UI:

from spec

job "fails_every_10" {

  ui {
    description = "A job that fails **every 10 allocations**, but a 1x [restart](https://nomadproject.io) and 1x <a href='https://hashicorp.com'>reschedule</a> means that a given allocation would have to fail 4x (initial, restarted, rescheduled, rescheduled+restarted) to be considered permanently failed, or 0.01% permanent failure rate (1/10,000 allocs)"
    link {
      label = "nomad"
      url = "https://nomadproject.io"
    }
    link {
      label = "Some Arbitrary String Label"
      url = "http://hashicorp.com/"
    }
  }

...

Notably, this supports markdown, so you can, for example, add a table with a jobspec like this:

job "ui-block-tester" {

  ui {
    description = <<EOT
| Name              | Team                 | Position | Average | HRs | Wikipedia Article                                          |
|-------------------|----------------------|----------|---------|-----|------------------------------------------------------------|
| Babe Ruth         | New York Yankees     | OF/P     | .342    | 714 | [Link](https://en.wikipedia.org/wiki/Babe_Ruth)            |
| Hank Aaron        | Atlanta Braves       | OF       | .305    | 755 | [Link](https://en.wikipedia.org/wiki/Hank_Aaron)           |
| Ted Williams      | Boston Red Sox       | OF       | .344    | 521 | [Link](https://en.wikipedia.org/wiki/Ted_Williams)         |
| Jackie Robinson   | Brooklyn Dodgers     | 2B       | .311    | 137 | [Link](https://en.wikipedia.org/wiki/Jackie_Robinson)      |
| Roberto Clemente  | Pittsburgh Pirates   | OF       | .317    | 240 | [Link](https://en.wikipedia.org/wiki/Roberto_Clemente)     |
EOT
    link {
      label = "nomad"
      url = "https://nomadproject.io"
    }
    link {
      label = "Some Arbitrary String Label"
      url = "http://hashicorp.com/"
    }
  }

...

Resolves #18127
Resolves #15706

[github-actions]

Ember Test Audit comparison

	main	da068a8	change
passes	1559	1562	+3
failures	0	0	0
flaky	0	0	0
duration	11m 21s 907ms	11m 20s 165ms	-01s 742ms

[octdanb]

This is a great proposal, +1

[tgross]

Weirdly no, because there's an empty JobUIConfig block here, which the new job populates.

[tgross]

This reads strangely but still works the same either way. After we do *nj = *j, then nj.UI and j.UI are pointing to the same location in memory. By doing nj.UI = nj.UI.Copy() we're copying that object to a new memory location and assigning that pointer to nj.UI but not j.UI.

For readability, it does feel like it'd make more sense to use j as the receiver for all 5 of these deep copy lines, but it's harmless.

[david-yu]

Wondering what it would take to perhaps have a config flag that gates this functionality for the UI?

[philrenaud]

@david-yu I believe a sentinel policy could target this block.

[david-yu]

@david-yu I believe a sentinel policy could target this block.

That could possibly work. May be a good suggestion for a template to add?

[tgross]

Last item on the pointer on JobUIConfig.Links and this should be good-to-go

[tgross]

LGTM!

Juanita is out and I've verified the PR addresses here concerns