Greenfield API: Get current User

Builds on #1368
This PR adds a new endpoint: Get current user.. It only returns the current user's id and email for now( let's extend later)
It also adds a new permission: `ProfileManagement` which is needed for this endpoint (and for update endpoints later)

BTCPayServer/Controllers/RestApi/Users/UsersController.cs

@@ -0,0 +1,47 @@
+using System.Threading.Tasks;
+using BTCPayServer.Client.Models;
+using BTCPayServer.Data;
+using BTCPayServer.Hosting.OpenApi;
+using BTCPayServer.Security;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Identity;
+using Microsoft.AspNetCore.Mvc;
+using NSwag.Annotations;
+
+namespace BTCPayServer.Controllers.RestApi.Users
+{
+    [ApiController]
+    [IncludeInOpenApiDocs]
+    [OpenApiTags("Users")]
+    [Authorize(AuthenticationSchemes = AuthenticationSchemes.ApiKey)]
+    public class UsersController : ControllerBase
+    {
+        private readonly UserManager<ApplicationUser> _userManager;
+
+        public UsersController(UserManager<ApplicationUser> userManager)
+        {
+            _userManager = userManager;
+        }
+    
+        [OpenApiOperation("Get current user information", "View information about the current user")]
+        [SwaggerResponse(StatusCodes.Status200OK, typeof(ApiKeyData),
+            Description = "Information about the current user")]
+        [Authorize(Policy = Policies.CanModifyProfile.Key, AuthenticationSchemes = AuthenticationSchemes.ApiKey)]
+        [HttpGet("~/api/v1/users/me")]
+        public async Task<ActionResult<ApplicationUserData>> GetCurrentUser()
+        {
+            var user = await _userManager.GetUserAsync(User);
+            return FromModel(user);
+        }
+        
+        private static ApplicationUserData FromModel(ApplicationUser data)
+        {
+            return new ApplicationUserData()
+            {
+                Id = data.Id,
+                Email = data.Email
+            };
+        }
+    }
+}