Add support to install and update PowerShell plugins

lib/core/framework/Install-IcingaFrameworkPlugins.psm1

@@ -0,0 +1,98 @@
+function Install-IcingaFrameworkPlugins()
+{
+    param(
+        [string]$PluginsUrl
+    );
+
+    $ProgressPreference = "SilentlyContinue";
+    $Tag                = 'Unknown';
+
+    if ([string]::IsNullOrEmpty($PluginsUrl)) {
+        if ((Get-IcingaAgentInstallerAnswerInput -Prompt 'Do you provide a custom repository for the Icinga Plugins?' -Default 'n').result -eq 1) {
+            $branch = (Get-IcingaAgentInstallerAnswerInput 'Which version to you want to install? (snapshot/stable)' -Default 'v' -DefaultInput 'stable').answer
+            if ($branch.ToLower() -eq 'snapshot') {
+                $PluginsUrl  = 'https://github.com/Icinga/icinga-powershell-plugins/archive/master.zip';
+            } else {
+                $LatestRelease = (Invoke-WebRequest -Uri 'https://github.com/Icinga/icinga-powershell-plugins/releases/latest' -UseBasicParsing).BaseResponse.ResponseUri.AbsoluteUri;
+                $PluginsUrl    = $LatestRelease.Replace('/releases/tag/', '/archive/');
+                $Tag           = $PluginsUrl.Split('/')[-1];
+                $PluginsUrl    = [string]::Format('{0}/{1}.zip', $PluginsUrl, $Tag);
+
+                $CurrentVersion = Get-IcingaPowerShellModuleVersion 'icinga-powershell-plugins';
+
+                if ($null -ne $CurrentVersion -And $CurrentVersion -eq $Tag) {
+                    Write-Host 'Your Icinga Plugins are already up-to-date';
+                    return @{
+                        'PluginUrl' = $PluginsUrl
+                    };
+                }
+            }
+        } else {
+            $PluginsUrl = (Get-IcingaAgentInstallerAnswerInput -Prompt 'Please enter the full path to your Icinga Plugin repository' -Default 'v').answer;
+        }
+    }
+
+    $ModuleDirectory = Get-IcingaFrameworkRootPath;
+    $DownloadPath    = (Join-Path -Path $ENv:TEMP -ChildPath 'icinga-powershell-plugins.zip');
+    Write-Host ([string]::Format('Downloading Icinga Plugins into "{0}"', $DownloadPath));
+
+    Invoke-WebRequest -UseBasicParsing -Uri $PluginsUrl -OutFile $DownloadPath;
+
+    Write-Host ([string]::Format('Installing plugins into "{0}"', ($ModuleDirectory)));
+    Expand-IcingaZipArchive -Path $DownloadPath -Destination $ModuleDirectory | Out-Null;
+
+    $FolderContent = Get-ChildItem -Path $ModuleDirectory;
+    $Extracted     = '';
+
+    foreach ($entry in $FolderContent) {
+        if ($entry -eq 'icinga-powershell-plugins') {
+            # Skip the plugins directory directly
+            continue;
+        }
+        if ($entry -like 'icinga-powershell-plugins-*') {
+            $Extracted = $entry;
+        }
+    }
+
+    if ([string]::IsNullOrEmpty($Extracted)) {
+        Write-Host 'No update package could be found.'
+        return @{
+            'PluginUrl' = $PluginsUrl
+        };
+    }
+
+    $NewDirectory = (Join-Path -Path $ModuleDirectory -ChildPath 'icinga-powershell-plugins');
+    $ExtractDir   = (Join-Path -Path $ModuleDirectory -ChildPath $Extracted);
+    $BackupDir    = (Join-Path -Path $ExtractDir      -ChildPath 'previous');
+    $OldBackupDir = (Join-Path -Path $NewDirectory    -ChildPath 'previous');
+
+    if ((Test-Path $NewDirectory)) {
+        Write-Host 'Creating backup directory';
+        if ((Test-Path $OldBackupDir)) {
+            Write-Host 'Importing old backups into new module version...';
+            Move-Item -Path $OldBackupDir -Destination $ExtractDir;
+        } else {
+            Write-Host 'No previous backups found. Creating new backup space';
+            if ((Test-Path $BackupDir) -eq $FALSE) {
+                New-Item -Path $BackupDir -ItemType Container | Out-Null;
+            }
+        }
+        Write-Host 'Moving old module into backup directory';
+        Move-Item -Path $NewDirectory -Destination (Join-Path -Path $BackupDir -ChildPath (Get-Date -Format "MM-dd-yyyy-HH-mm-ffff"));
+    }
+
+    Write-Host ([string]::Format('Installing new module version "{0}"', $Tag));
+    Start-Sleep -Seconds 2;
+    Move-Item -Path (Join-Path -Path $ModuleDirectory -ChildPath $Extracted) -Destination $NewDirectory;
+
+    Unblock-IcingaPowerShellFiles -Path $NewDirectory;
+    # In case the plugins are not installed before, load the framework again to
+    # include the plugins
+    Use-Icinga;
+
+    Write-Host 'Icinga Plugin update has been completed';
+
+    return @{
+        'PluginUrl' = $PluginsUrl
+    };
+}