Support GitLab "System Hook" push webhooks for ApplicationSets

[eadred]

Summary

Similar to the issue that was reported in #9625, the ApplicationSet controller does not support Gitlab System Hook events - instead it will return a 400 response. Although the fix for 9625 added system hook support for Applications, the equivalent was not done for ApplicationSets.

Motivation

As with #9625, on a company GItlab instance it would be easier to be able to configure a system wide webhook centrally for ApplicationSets.

Proposal

Implement the same solution as for Applications (#12547), except on this line:

argo-cd/applicationset/webhook/webhook.go

Line 167 in 522d07a

payload, err = h.gitlab.Parse(r, gitlab.PushEvents, gitlab.TagEvents, gitlab.MergeRequestEvents)

ie include gitlab.SystemHookEvents in the params.

[conrity]

We need this too, we would like to use webhook to trigger gitlab reconcile after new branch created.

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: preview-appset
  namespace: argocd
spec:
  goTemplate: true
  goTemplateOptions: ["missingkey=error"]
  generators:
  - scmProvider:
      gitlab:
        # The base GitLab group to scan.  You can either use the group id or the full namespaced path.
        group: "backend"
        # For self-hosted GitLab:
        api: https://gitlab.mycompany.com/
        # If true, scan every branch of every repository. If false, scan only the default branch. Defaults to false.
        allBranches: true
        # If true, recurses through subgroups. If false, it searches only in the base group. Defaults to false.
        includeSubgroups: true
        # If true and includeSubgroups is also true, include Shared Projects, which is gitlab API default.
        # If false only search Projects under the same path. Defaults to true.
        includeSharedProjects: true
        # Reference to a Secret containing an access token. (optional)
        tokenRef:
          secretName: gitlab-token
          key: token
      filters:
      - repositoryMatch: web-socket-service
        branchMatch: "^[a-zA-Z]+-[0-9]+.*$"

We have appset that generates feature-branches application, but refresh without webhook takes too much time.