Add secret provider functionality and enhance configuration data resolution

Private/Register-ConfigurationDataSecretProvider.ps1

@@ -0,0 +1,40 @@
+function Register-ConfigurationDataSecretProvider {
+    [CmdletBinding()]
+    Param(
+        [Parameter(Mandatory=$true)]
+        [string]
+        $Name,
+
+        [Parameter(Mandatory=$true)]
+        [string[]]
+        $SupportedTypes,
+
+        [Parameter(Mandatory=$true)]
+        [scriptblock]
+        $Resolver
+    )
+
+    if([string]::IsNullOrWhiteSpace($Name)){
+        throw "Secret provider [Name] must not be empty."
+    }
+
+    if($SupportedTypes.Count -eq 0){
+        throw "Secret provider [$Name] must define at least one supported type."
+    }
+
+    foreach($Type in $SupportedTypes){
+        if($Type -notin @("credential", "securestring", "string")){
+            throw "Secret provider [$Name] uses unsupported type [$Type]."
+        }
+    }
+
+    if($null -eq $script:ConfigurationDataSecretProviders){
+        $script:ConfigurationDataSecretProviders = @{}
+    }
+
+    $script:ConfigurationDataSecretProviders[$Name.ToLowerInvariant()] = [PSCustomObject]@{
+        Name = $Name
+        SupportedTypes = @($SupportedTypes | ForEach-Object { $_.ToLowerInvariant() })
+        Resolver = $Resolver
+    }
+}