Add `incident declare` slash command with base logic

What does this MR do and why?

Adds base code for incident declare slash command.

This change is behind a feature flag and is disabled by default.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Follow the steps mentioned in https://gitlab.com/gitlab-org/manage/integrations/team/-/blob/main/integrations/slack.md for setting up a slack workspace to test the command.
2. Enable the FF per user to see it on the list of commands if that specific user sends a command.
3. Try using /dev-gitlab help to check the list of commands. If FF is disabled, it does not show /dev-gitlab incident declare in the list of commands. If the FF is enabled, then it shows up.
4. Try using /dev-gitlab incident declare. If FF is disabled, it shows that the command is invalid, if FF is enabled, it shows It works! in the slack message.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #344856 (closed)

config/feature_flags/development/incident_declare_slash_command.yml

+---
+name: incident_declare_slash_command
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/101177
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/378072
+milestone: '15.6'
+type: development
+group: group::respond
+default_enabled: false