feat: support http coder urls (#217)

Currently, in the Login Form, we reject non-https URLs. When this HTTPS requirement was added, the Coder Desktop app was unable to make HTTP requests, as it did not have `NSArbitraryLoads`. We ended up needing to enable that flag in order to use the Agent API. (`http://workspace.coder:4`) To support Coder deployments behind VPNs without HTTPS, we can now loosen the requirement. With this PR, the behavior [matches Windows.](https://github.com/coder/coder-desktop-windows/blob/ac22fe4c44dea03b21557b4a0f1e214397bc3ea4/App/Services/CredentialManager.cs#L175-L176)

Author: ethanndickson

Coder-Desktop/Coder-Desktop/Views/LoginForm.swift

@@ -194,8 +194,8 @@ func validateURL(_ url: String) throws(LoginError) -> URL{
 guardlet url =URL(string: url)else{
 throw.invalidURL
 }
-guard url.scheme =="https"else{
-throw.httpsRequired
+guard url.scheme =="https"|| url.scheme =="http"else{
+throw.invalidScheme
 }
 guard url.host !=nilelse{
 throw.noHost
@@ -204,7 +204,7 @@ func validateURL(_ url: String) throws(LoginError) -> URL{
 }
 
 enumLoginError:Error{
-casehttpsRequired
+caseinvalidScheme
 case noHost
 case invalidURL
 case outdatedCoderVersion
@@ -213,12 +213,12 @@ enum LoginError: Error{
 
 vardescription:String{
 switchself{
-case.httpsRequired:
-"URL must use HTTPS"
+case.invalidScheme:
+"Coder URL must use HTTPS or HTTP"
 case.noHost:
-"URL must have a host"
+"Coder URL must have a host"
 case.invalidURL:
-"Invalid URL"
+"Invalid Coder URL"
 case.outdatedCoderVersion:
 """
  The Coder deployment must be version \(Validator.minimumCoderVersion)