feat: adjust ThingFilter and ThingDiscovery interfaces

[JKRhb]

This PR contains the changes to the ThingFilter and ThingDiscovery interfaces that have originally been part of #405.

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[danielpeintner]

Does it make sense to state why you suggest both changes...

w.r.t. url

I think I see the reasoning..

direct and the directory methods need URLs in order to work

w.r.t. to require ThingFilter

Is it because one can pass in null anyway?
Did we check all algorithms?

[JKRhb]

Thank you for your comment, @danielpeintner, you are right, the PR needs a bit more clarification 😅 The change of the ThingFilter was related to making the URL mandatory. I think the algorithms do indeed need to be adjusted, I did not manage to revisit them yet.

[danielpeintner]

Call 2022-10-17

• @relu91 overall discussion, see Possible alternative design of Discovering API #222
  • @JKRhb separate functions make sense
  • @zolkis need to understand proposal better w.r.t. to function calls
• @JKRhb still need to check algorithms (@zolkis start() and next() method changes need work)
• @zolkis I am against the optional/required changes
• @zolkis suggest to experiments with the ideas we have (@JKRhb @relu91 agree)

[danielpeintner]

Closing since we move on with #364