Fix bug with trailing slashes in URL

[inahga]

In #1579 (comment) I identified cases where https://example.com would be encoded as https://example.com/ (notice the trailing slash). At the time I didn't think this was relevant, but it turns out to affect task ID derivation.

Task ID derivation is sha256sum(encoded_task_config). If we are peered with a non-janus aggregator and if we introduce trailing slashes, Janus could derive a different task ID than its peer. (This is actually somewhat unlikely since we'd derive task ID directly from the encoded bytes, but is a subtle inconsistency that could hit us later).

Instead, provide TryInto<url::Url> to ease converting DAP Urls to a more useful type.

[inahga]

I also snuck in a change to implement a custom Debug for Url, since otherwise it'll give us the unhelpful bytewise output, I could use a quick look at that.