add public api for observed address with multipath

[divagant-martian]

Some changes that are relevant in this PR:

• protocol wise changes are minimal since it was clear from the beginning that observed addresses were meant to be kept by path.
• We still send observed address reports with path challenges and responses. This was originally intended but now results on double reports when opening a path. This seems fine to me for now since helps reports to remain updated. We could evaluate removing this
• we keep track of whether the path has been informed as open to the application to emit the relevant events afterwards. We should probably use this for status as well if we continue with emitting the event when the path is in fact open (a packet has been successfully received)
• Address updates can be obtained via the Path api
• I noticed we might be missing a wake in processing events somwhere, there is a noticeable delay in the test, unrelated to address discovery as far as I can tell
• I had this using tokio, like one usually does, but then wasm failed due to the missing runtime future. Since tokio can't be used to spawn tasks, using instead the Runtime trait, there are some features I was using that are no longer available, like AbortHandles. We could iterate over this to have better task cancellation on drop, maybe terminate the stream on closing/abandoning etc
• CI failures are unrelated