feat: add time helpers

[freak12techno]

Fixes #3717.

The humanizeDirection helper is 100% copied from Prometheus, the since is added by me.
The tests are also copied from Prometheus and adapted, plus I added my own since test.

Tested it locally: creating an alert like this one:

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

with a template like this one

{{ 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" }}

Labels:
{{ range .Labels.SortedPairs }} - {{ .Name }} = {{ .Value }}
{{ end }}
<a href="{{ .GeneratorURL }}">Source</a>

{{ end }}
{{ end }}

produces the following output:

🔥 Alerts Firing

🖥️ Host: test (test3)
🟠 Severity: warning
📖 Alert: aaa
ℹ️ Details: 
⌛ Firing since: 2024-02-13 22:22:49.535404 +0300 MSK m=+4.535759668 (for 10.01s)
📝 Description: 

Labels:
 - alertname = aaa
 - host = test
 - instance = test3

Source

[freak12techno]

tbh I am not sure how to test this one properly without mocking time.Since, which as far as I know cannot be done without some kinds of workaround, this one looks ugly and flaky (as it relies on local time and will fail if the time between creating this variable and rendering is more than 1 second), but it kinda works, let me know if there's a better way to do it

[freak12techno]

@gotjosh can you review or ping somebody to do that?

[gotjosh]

@freak12techno how do you feel about moving the helper into https://github.com/prometheus/common so that we don't have to copy/pate here?

[freak12techno]

@gotjosh I am ok with this, I can do the following:

1. make a PR on prometheus/common moving what I wrote here to there
2. make a PR on this repo utilizing prometheus/common methods
3. make a PR on prometheus/prometheus deleting the code from there and utilizing the one on prometheus/common

does this sound decent?

[gotjosh]

Sounds perfect to me!

[freak12techno]

I'll close this one as it'll be required to rewrite it anyways.