Slack: Use chat.postMessage API by default (#32511)

* Slack: Use only chat.postMessage API

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Slack: Check for response error

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Slack: Support custom webhook URL

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Simplify

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Fix tests

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Rewrite tests to use stdlib

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update pkg/services/alerting/notifiers/slack.go

Co-authored-by: Dimitris Sotirakis <sotirakis.dim@gmail.com>

* Clarify URL field name

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Fix linting issue

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Fix test

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Fix up new Slack notifier

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Improve tests

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Fix lint

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Slack: Make token not required

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Alerting: Send validation errors back to client

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Document how token is required

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Make recipient required when using Slack API

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

* Fix field description

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

Co-authored-by: Dimitris Sotirakis <sotirakis.dim@gmail.com>

pkg/services/provisioning/notifiers/testdata/test-configs/incorrect-settings/incorrect-settings.yaml

@@ -1,10 +1,9 @@
 notifiers:
-  - name: slack-notification-without-url-in-settings
+  - name: slack-notification-without-token-in-settings
     type: slack
     org_id: 2
     uid: notifier1
     is_default: true
     settings:
       recipient: "XXX"
-      token: "xoxb"
-      uploadImage: true
+      uploadImage: true