[JS] Add Server Sent Events (SSE) implementation for Eclipse Dittos events

[ffendt]

Currently the JS client only allows receiving events via WebSocket. We should also provide an implementation that uses the existing SSE API that is already provided by Eclipse Ditto.

Note: Main reason for this not being in the initial commit is, that the event-source-polyfill seemed to not work in web components that used the client. When implementing SSE, definitely test if the implementation works inside web components (e.g. a StencilJS web component)