Add mattermost integration resource

#607 Add mattermost integration.

It's a copy and paste and adapt of the gitlab_integration_slack files. I used the order in the api documentation https://docs.gitlab.com/ee/api/integrations.html#createedit-mattermost-notifications-integration

curl --request PUT \
  --url https://gitlab.staging.mycompany.io/api/v4/projects/42/integrations/mattermost \
  --header 'Content-Type: multipart/form-data' \
  --header 'PRIVATE-TOKEN: myapiapireadsudotoken' \
  --header 'Sudo: root' \
  --form channel=mytestchannel \
  --form webhook=https://mattermost.mycompany.com/hooks/secrethooktoken

produces

{
	"id": 1,
	"title": "Mattermost notifications",
	"slug": "mattermost",
	"created_at": "2023-08-29T10:58:24.413Z",
	"updated_at": "2023-08-29T11:19:10.734Z",
	"active": true,
	"commit_events": true,
	"push_events": true,
	"issues_events": true,
	"incident_events": false,
	"confidential_issues_events": true,
	"merge_requests_events": true,
	"tag_push_events": true,
	"note_events": true,
	"confidential_note_events": true,
	"pipeline_events": true,
	"wiki_page_events": true,
	"job_events": true,
	"comment_on_event_enabled": true,
	"properties": {
		"username": null,
		"notify_only_broken_pipelines": null,
		"branches_to_be_notified": null,
		"labels_to_be_notified": null,
		"labels_to_be_notified_behavior": null,
		"push_channel": null,
		"issue_channel": null,
		"confidential_issue_channel": null,
		"merge_request_channel": null,
		"note_channel": null,
		"confidential_note_channel": null,
		"tag_push_channel": null,
		"pipeline_channel": null,
		"wiki_page_channel": null,
		"deployment_channel": null,
		"incident_channel": null
	}
}

which leads me to the conclusion that the channel property does not exist on GitLab side, even though it is implemented by go-gitlab and mentioned in the GitLab api documentation here https://docs.gitlab.com/ee/api/integrations.html#createedit-mattermost-notifications-integration.

confidential_note_channel and note_channel seem to be treated as the same property.