Adds DuplicatiServerClient for API interaction

Introduces a `DuplicatiServerClient` to interact with the Duplicati server API, including authentication and various V1/V2 endpoints.

Adds `ServerCredentialType` to specify the authentication mechanism (Password or Token).

Includes a usage example to demonstrate client capabilities.

WebserverCore.Client.UsageExample/WebserverCore.Client.UsageExample.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net8.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <ProjectReference Include="..\Duplicati\WebserverCore\Duplicati.WebserverCore.csproj" />
+    </ItemGroup>
+
+</Project>