feat: add time helpers

[freak12techno]

Closes #3717.

Bumps prometheus/common to latest, imports HumanizeDuration from it and exposes it and time.Since as template methods.
Also added some tests for templating it (which is mostly copypaste from Prometheus repo).

Here's how the alert would look like:

Template part (the one specifying how to render):

{{ define "alerts_list" }}
{{ range . }}{{ if index .Labels "host" -}}
🖥️ Host: {{ index .Labels "host" }} ({{ index .Labels "instance" }})
{{- else -}}
🖥️ Host: {{ index .Labels "instance" }}
{{- end }}
{{ if eq (index .Labels "severity") "critical" -}}
🔴 Severity: critical
{{- else -}}
🟠 Severity: warning
{{- end }}
📖 Alert: {{ index .Labels "alertname" }}
ℹ️ Details: {{ index .Annotations "summary" }}
⌛ Firing since: {{ .StartsAt }} (for {{ .StartsAt | since | humanizeDuration }})
📝 Description: {{ index .Annotations "description" }}

Alert generation:

./amtool alert add --alertmanager.url http://localhost:9093 alertname=aaa host=test instance=test3

Alert received in Telegram (the "Starts at" thing was displayed this way before and I haven't touched it; main thing is the "for 10s" block):

[freak12techno]

@gotjosh mind looking at it?

[gotjosh]

LGTM

Thank you so much for your hard work.

[gotjosh]

Can you please fix the go.mod?

[freak12techno]

@gotjosh done

[freak12techno]

(I checked the CI failures, all of them shouldn't be caused by my changes, but can someone please confirm this?)

[gotjosh]

Can you rebase?

[freak12techno]

@gotjosh it says it's up to date

[gotjosh]

@freak12techno how do you feel about bringing humanize timestamp from Prometheus as well? It feels like it could be useful per your message above.

[freak12techno]

@gotjosh sounds like a nice idea, but it'll require another bump of prometheus/common. I suggest moving it to another PR, which I will also do, is it ok?

[gotjosh]

Correct, I was not suggesting as part of this PR. I'll merge this right now - thanks very much for your contributions, highly appreciated.