fix(iroh-relay): send correct Proxy-Authorization credentials

[fluffypony]

Description

Fixes #4561.

Now using data_encoding::BASE64 instead of BASE64URL for the Proxy-Authorization: Basic header in dial_url_proxy.

While writing the test for this, it turned out that there was a second problem in the header, related to the fact that Url::username() and Url::password() return the percent-encoded version. So if you had a password like s3cret>> (from the issue), it would be sending s3cret%3E%3E to the proxy, and the alphabet wasn't even the problem. Ditto any other character that has to be escaped in URLs, like @ or :. The second commit in this PR percent-decodes the username and password before encoding them. This matches the behavior of what reqwest does when given the same proxy URL, at least as far as non-invalid-UTF-8 characters go. And since #4463 the net_report probes will be sending that URL to reqwest, so there can currently be a mismatch between what the probes are sending and what we'd send when connecting to the relay.

This makes percent-encoding a direct dependency of iroh-relay. It's already an indirect one through url.

Both of the tests added in this PR (which dial to a fake proxy and check the header it got) fail on main.

API Changes

n/a

Notes & open questions

Note that the percent-decoding goes a little beyond what we talked about in the issue. If you want, we can split that out into a different PR. It does change the behavior for literal % signs followed by two hex characters in the password, which will now get decoded. Those would have to be written as %25, which matches the behavior of curl and reqwest.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All API changes documented.

[flub]

thanks! looks good i think