Add channel test providers for Email, Slack, Teams, and Webhook

- Implemented EmailChannelTestProvider to generate email preview payloads.
- Implemented SlackChannelTestProvider to create Slack message previews.
- Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews.
- Implemented WebhookChannelTestProvider to create webhook payloads.
- Added INotifyChannelTestProvider interface for channel-specific preview generation.
- Created ChannelTestPreviewContracts for request and response models.
- Developed NotifyChannelTestService to handle test send requests and generate previews.
- Added rate limit policies for test sends and delivery history.
- Implemented unit tests for service registration and binding.
- Updated project files to include necessary dependencies and configurations.

etc/authority.yaml.sample

@@ -52,7 +52,7 @@ bootstrap:
 # against the application content root, enabling air-gapped deployments
 # that package plug-ins alongside binaries.
 pluginDirectories:
-  - "../PluginBinaries/Authority"
+  - "../StellaOps.Authority.PluginBinaries"
   # "/var/lib/stellaops/authority/plugins"
 
 # Plug-in manifests live in descriptors below; per-plugin settings are stored
@@ -73,7 +73,7 @@ plugins:
       metadata:
         defaultRole: "operators"
     # Example for an external identity provider plugin. Leave disabled unless
-    # the plug-in package exists under PluginBinaries/Authority.
+    # the plug-in package exists under StellaOps.Authority.PluginBinaries.
     ldap:
       type: "ldap"
       assemblyName: "StellaOps.Authority.Plugin.Ldap"