Implement forceAsItem-option

[timbms]

Discussed in https://github.com/openhab/openhab-ios/discussions/667

^{Originally posted by Flole998 April 19, 2022} This is part of https://github.com/openhab/openhab-core/issues/2185.

In https://github.com/openhab/openhab-core/pull/2657 an option to force charts for groups to be rendered as value of the group instead of rendering all members of the group in a chart was implemented. So what is left here now is changing how the chartUrl is built if that option is passed and is true so that it is rendered in the app the same way as in the web-ui. That option probably needs to be added to Endpoint.swift. I think this search for the legend-parameter shows pretty much all changes necessary for the new parameter. Unfortunately I can't test this, so I would be working blind and I would appreciate if someone with more experience (meaning: any experience) and the ability to test this could do this.

In the web-ui this was implemented with https://github.com/openhab/openhab-webui/pull/1256, the same logic needs to be added to the app.