feat(alerting): allow configuring the topic for Zulip alerts (#1764)

Zulip alerts previously always used a hardcoded "Gatus" topic, sending
every alert and resolution into a single thread. Add a "topic" config
field (defaulting to "Gatus" for backwards compatibility) that supports
the [ENDPOINT_NAME], [ENDPOINT_GROUP] and [ALERT_DESCRIPTION] placeholders,
matching the templating convention used by other providers such as clickup.

Closes #1387

Co-authored-by: Chris (ChrisJr404) <11917633+ChrisJr404@users.noreply.github.com>
Co-authored-by: TwiN <twin@linux.com>

README.md

@@ -2530,6 +2530,7 @@ endpoints:
 | `alerting.zulip.bot-api-key`       | Bot API key                                                                         | Required `""` |
 | `alerting.zulip.domain`            | Full organization domain (e.g.: yourZulipDomain.zulipchat.com)                      | Required `""` |
 | `alerting.zulip.channel-id`        | The channel ID where Gatus will send the alerts                                     | Required `""` |
+| `alerting.zulip.topic`             | The topic under which the alerts will be sent.<br />Supports the `[ENDPOINT_NAME]`, `[ENDPOINT_GROUP]` and `[ALERT_DESCRIPTION]` placeholders. | `Gatus`       |
 | `alerting.zulip.overrides`         | List of overrides that may be prioritized over the default configuration            | `[]`          |
 | `alerting.zulip.overrides[].group` | Endpoint group for which the configuration will be overridden by this configuration | `""`          |
 | `alerting.zulip.overrides[].*`     | See `alerting.zulip.*` parameters                                                   | `{}`          |
@@ -2541,6 +2542,7 @@ alerting:
     bot-api-key: "********************************"
     domain: some.zulip.org
     channel-id: 123456
+    topic: "[ENDPOINT_NAME]" # Optional; groups each endpoint's alerts under its own topic. Defaults to "Gatus"
 
 endpoints:
   - name: website