[Embeddable Rebuild] Make HasSupportedTriggers optional for drilldown actions

[Heenawter]

Describe the feature:
Currently, we require supportedTriggers() to be defined in both the create and edit drilldown actions in order for them to be compatible. This is overkill IMO, because it's possible for drilldowns to only be supported via the context menu, which is already provided as a default:

kibana/x-pack/plugins/dashboard_enhanced/public/services/drilldowns/actions/flyout_create_drilldown/flyout_create_drilldown.tsx

Lines 116 to 119 in e3027cf

	const triggers = [
	...ensureNestedTriggers(embeddable.supportedTriggers()),
	CONTEXT_MENU_TRIGGER,
	];

Because it's required, embeddables that don't need other supported triggers are forced to return an empty array in order to have context menu drilldowns. See #201870 (comment) for more context.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)