feat(dashboards-ng): event bus for widget communication

[chintankavathia]

Introduces EventBus service to emit and subscribe events with data between widgets.
Also introduces getEventBusInstance util with separate entry point @siemens/dashboards-ng/event-bus to allow non angular widgets to utilize same service.

Usage

// to subscribe to an event
eventBus = inject(EventBus);

eventBus.on('languageChange').subscribe(lang => console.log(lang));

// to emit an event
eventBus.emit('languageChange', 'de');

// to access current snapshot of events

eventBus.snapshot(); // returns whole snapshots of all events

eventBus.snapshot('languageChange');  // returns only `languageChange` specific event snapshot

eventBus.snapshot('filter', ['location']); // returns `filter` event snapshots having filter keys matching `location`


// EventBus with custom event types

type MyCustomEventType = {name: 'foo'; data: boolean} 

eventBus = inject(EventBus<MyCustomEventType>);

eventBus.on('foo');
eventBus.emit('foo', false);


// Using EventBus instance in non angular widgets (e.g react/vue)

import {getEventBusInstance} from '@siemens/dashboards-ng/event-bus';

eventBus = getEventBusInstance();

---

Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request introduces a well-designed SiEventBus service for inter-widget communication, which is a significant and valuable feature. The implementation thoughtfully supports both Angular and non-Angular contexts by leveraging shared state on the window object, making it suitable for microfrontend architectures. The documentation and tests are comprehensive. My review includes suggestions to improve type safety by adhering to the project's style guide.

[spliffone]

Can we potentially inject the window which is better testable and type-safe? Further we could consider to use the indexdb as store which is persistemt.

[chintankavathia]

No we cannot use any angular specific here as we need this to be used by non angular runtimes as well.
indexDb wouldn't be good choice here as we need synchronous data while indexDb is async moreover we don't want to persist data on page reload.

[spliffone]

I would prefer the provider approach instead of a window + singleton mechanism or is this not possible?

[chintankavathia]

provider approach cannot be used as this is meant for only non angular widgets where it shouldn't import anything from @angular package

[spliffone]

From a security / privacy perspective I wouldn't provide a full list of all states at once - consumers should know the events they want to subscribe.

[spliffone]

We shouldn't expose the base class instead we should use interfaces

[spliffone]

Could we also add a usage chapter to the docs?

[chintankavathia]

currently our docs directly points to the readme for usage of flexible dashboards.