rpc: fix - Unix domain socket maddrs used with NewApi

[djdv]

closes: #10018

This simply removes the need to dial the domain by specifying the dialer in the transport.
And removes any concerns in regards to the socket's path containing characters that are not valid HTTP URLs.
We make requests to a made up domain http://unix which the client is happy to engage with.

There may be a better way to do this but I couldn't figure one out.
I considered trying to avoid calling manet.DialArgs since it will be called twice for non-UDS maddrs (once here and again in NewApiWithClient).
But if we do something like multiaddr.ValueForProtocol(multiaddr.P_UNIX), this will cause issues on platforms like Windows.
This is because multiaddr requires / delimiters (/unix/C:/somewhere), while net.Dial and the OS want C:\somewhere.
ValueForProtocol will return C:/somewhere while manet.DialArgs returns C:\somewhere.
So I just stuck with manet.Dial.

As an aside, I don't really understand why NewApiWithClient is converting from a multiaddr, to a string, then back to a multiaddr. But this is out of scope.

[djdv]

Some other commit was causing a conflict with the import statements, so I rebased and force pushed. No changes to the actual code in this PR though.