fix: validate server URL has a host

[ethanndickson]

Closes #53.

The linked issue was caused by a subtle difference in how we add paths to the server URLs in Go and Swift

In both languages, the (erroneous) url https:/dev.coder.com is parsed the same:

• Scheme: https
• Host: "" / nil
• Relative Path: /dev.coder.com

However, in Swift, when we transform a server URL into a specific route on that server, such as /api/v2/users/me, we don't replace any existing part of the relative path, we just append to it, producing http:/dev.coder.com/api/v2/users/me, which gets sent, and the server responds to, just fine.

In Go, when we do the same transformation, we call svrURL.Parse("/api/v2/users/me"), which replaces the existing relative path, /dev.coder.com, leading to the error in the linked issue, which has no dev.coder.com portion at all.

Previously, we were only ensuring that the link was a valid URL, and that the scheme was https.
Now, we'll require that all server URLs have a host component. This means http:/dev.coder.com fails to validate with a better error message.

[ethanndickson]

• fix: validate server URL has a host #57 👈 (View in Graphite)
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