Greenfield: Add file endpoints and upload (#6075)

* Greenfield: Add file endpoints and upload

- Endpoints for server files
- File upload using `multipart/form-data`

Closes #6074.

Can also be tested using cURL:

- `curl --location 'https://localhost:14142/api/v1/files' --header 'Authorization: token MY_API_TOKEN' --form 'file=@"LOCAL_FILEPATH"'`
- `curl --location 'https://localhost:14142/api/v1/users/me/picture' --header 'Authorization: token MY_API_TOKEN' --form 'file=@"LOCAL_FILEPATH"'`

* Revert UnresolvedUri changes

* Add upload for store logo

BTCPayServer.Client/BTCPayServerClient.Files.cs

@@ -0,0 +1,29 @@
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+using BTCPayServer.Client.Models;
+
+namespace BTCPayServer.Client;
+
+public partial class BTCPayServerClient
+{
+    public virtual async Task<FileData[]> GetFiles(CancellationToken token = default)
+    {
+        return await SendHttpRequest<FileData[]>("api/v1/files", null, HttpMethod.Get, token);
+    }
+
+    public virtual async Task<FileData> GetFile(string fileId, CancellationToken token = default)
+    {
+        return await SendHttpRequest<FileData>($"api/v1/files/{fileId}", null, HttpMethod.Get, token);
+    }
+
+    public virtual async Task<FileData> UploadFile(string filePath, string mimeType, CancellationToken token = default)
+    {
+        return await UploadFileRequest<FileData>("api/v1/files", filePath, mimeType, "file", HttpMethod.Post, token);
+    }
+
+    public virtual async Task DeleteFile(string fileId, CancellationToken token = default)
+    {
+        await SendHttpRequest($"api/v1/files/{fileId}", null, HttpMethod.Delete, token);
+    }
+}