add timeout parameter for webhook configs or http_config

[LeoQuote]

What did you do?

I set up a webhook for alertmanager to call, but it is a bit slow, alertmanager tried to call it but failed with timeout , and the retry makes it been sent multiple times, it could be better if I can set timeout.

What did you expect to see?

What did you see instead? Under which circumstances?

Environment

• System information:

  Linux 5.4.66-gentoo x86_64

• Alertmanager version:

  v0.19.0

• Prometheus version:

  insert output of prometheus --version here (repeat for each prometheus
  version in your cluster, if relevant to the issue)

• Alertmanager configuration file:

insert configuration here

• Prometheus configuration file:

insert configuration here (if relevant to the issue)

• Logs:

2021-07-29 00:26:17.157322500 level=error ts=2021-07-28T16:26:17.157Z caller=notify.go:372 component=dispatcher msg="Error on notify" err="Post https://xxx: context deadline exceeded" context_err="context deadline exceeded"
2021-07-29 00:26:17.157324500 level=error ts=2021-07-28T16:26:17.157Z caller=dispatch.go:266 component=dispatcher msg="Notify for alerts failed" num_alerts=7 err="Post https://xxx: context deadline exceeded"
2021-07-29 00:30:07.160750500 level=error ts=2021-07-28T16:30:07.160Z caller=notify.go:372 component=dispatcher msg="Error on notify" err="Post https://xxx: context deadline exceeded" context_err="context deadline exceeded"
2021-07-29 00:30:07.160753500 level=error ts=2021-07-28T16:30:07.160Z caller=dispatch.go:266 component=dispatcher msg="Notify for alerts failed" num_alerts=6 err="Post https://xxx: context deadline exceeded"