Event WebSocket: Add topic filter

[florian-h05]

I am currently working on the first step of migrating Main UI from SSE to WS for event tracking.

Looking at the SSE event tracking, the UI heavily relies on topic filtering, which is more versatile than the source and type filters currently provided by the event WS.

Topic filters allow to filter by Item name etc., and having them available would also allow the UI to easily support both SSE and WS (as WS is not working everywhere right now, e.g. openHAB cloud or some corporate firewalls) for some period.

@J-N-K WDYT? IIRC you authored the event WS.

[spacemanspiff2007]

Also a negative filter would be very nice (see #3602)
Possible duplicate: #3764

[florian-h05]

Oops, didn’t notice that existing issue.

[florian-h05]

I went ahead and created #4550, which adds this filter.