Introduce additional tag types for incidents

Proposal

In #336026 (closed), we're allowing users to define incident start/end times.

Let's build on this by allowing people to define additional timestamp types (bold items identified as most important, should we need to break this into multiple iterations):

• when the impact was detected (either by system or human) impact detected
• when the Incident Response process was engaged/started response initiated
• when the problem impact was stabilized/mitigated impact mitigated
• when root cause identified cause identified

Design

Add additional event types into the pre-existing event type dropdown. As with start/end time, when an event type has been selected, the comment field pre-populates with the selected tag name, and that text is editable. The saved tag displays as a badge on the timeline view. Here's how that looks, in practice:

Dropdown with additional options	Default text supplied in the comment field	Default text is editable	Tag displays as a badge on the timeline view

Figma file

Note - in #336026 (closed), the dropdown added is a single select dropdown. Here, we should introduce a multi-select dropdown, as it's possible people will want to add multiple tags to single events.