Implement basic RPC configuration API endpoint (#864)

* Implement basic RPC information endpoint with test data

* Implement configuration options for RPC

* Rename RpcConfiguration to RpcResponse

* Rider insists on modifying dataSources.xml

* Implement documentation and fix code quality/gitignore issues

* Use SerializableAttribute instead of UsedImplicitlyAttribute

* Add warning regarding nullable assets

* Prevent precision issues with application ID

* Check asset properties for null/whitespace and return null

Documentation/RPC.md

@@ -0,0 +1,38 @@
+# RPC (Rich Presence)
+
+Lighthouse supports Discord's [Rich Presence](https://discordapp.com/rich-presence) feature, which allows you
+to display your current activity in Lighthouse to your Discord friends.
+
+This can be done by utilizing a pre-existing RPC client, such as [this one](https://github.com/LBPUnion/PLRPC),
+or by creating your own. This page will explain how to utilize the `/api/v1/rpc` endpoint to access the
+required information.
+
+## Authentication
+
+This endpoint does not require any form of authentication and is available to use without restriction, aside
+from any server admin imposed rate limiting.
+
+## Endpoints
+
+### GET `/api/v1/rpc`
+
+Returns a JSON object containing the following information:
+
+- `applicationId`: The ID of the Discord application.
+- `partyIdPrefix`: The prefix to use for the party ID. This is used to prevent collisions between
+  multiple instances of Lighthouse running on the same Discord application.
+- `usernameType`: Some compatible APIs require usernames instead of user IDs. A return value of `0`
+  indicates that usernames should be used, while a return value of `1` indicates that user
+  IDs should be used.
+- `assets`: A JSON object containing the following information:
+    - `podAsset`: Asset used when in the Pod.
+    - `moonAsset`: Asset used when creating on the Moon.
+    - `remoteMoonAsset`: Asset used when creating on a remote Moon.
+    - `developerAsset`: Asset used when playing a story mode level.
+    - `developerAdventureAsset`: Asset used when playing an adventure level.
+    - `dlcAsset`: Asset used when playing a DLC level.
+    - `fallbackAsset`: Asset used when client can't determine the slot type.
+
+> **Warning**
+> All `assets` properties are nullable and will return `null` if not set in configuration. Be sure to account
+> for this when using the returned data.