WebAPI for database integrity added

Restructure the solution
* The project has been split into the database integration "src/webapi" and the Blazor wasm project "src/y4f".

WebAPI
* API with CRUD capabilities

* The main task of the "WebAPI" project is to access the database data and make it available to the Blazor-WASM project via an API.

* GET example of the allergy table via the file "TestFetchAllergienData.razor", which can be tested via the browser "/TestFetchAllergienData".

src/WebApi/Controllers/KundenController.cs

@@ -0,0 +1,138 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using WebApi.Data;
+using WebApi.Models;
+
+namespace WebApi.Controllers
+{
+    [Route("api/[controller]")]
+    [ApiController]
+    public class KundenController : ControllerBase
+    {
+        private readonly WebApiContext _context;
+
+        public KundenController(WebApiContext context)
+        {
+            _context = context;
+        }
+
+        // GET: api/Kunden
+        [HttpGet]
+        public async Task<ActionResult<IEnumerable<Kunde>>> GetKundes()
+        {
+          if (_context.Kundes == null)
+          {
+              return NotFound();
+          }
+            return await _context.Kundes.ToListAsync();
+        }
+
+        // GET: api/Kunden/5
+        [HttpGet("{id}")]
+        public async Task<ActionResult<Kunde>> GetKunde(int id)
+        {
+          if (_context.Kundes == null)
+          {
+              return NotFound();
+          }
+            var kunde = await _context.Kundes.FindAsync(id);
+
+            if (kunde == null)
+            {
+                return NotFound();
+            }
+
+            return kunde;
+        }
+
+        // PUT: api/Kunden/5
+        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
+        [HttpPut("{id}")]
+        public async Task<IActionResult> PutKunde(int id, Kunde kunde)
+        {
+            if (id != kunde.Idkunde)
+            {
+                return BadRequest();
+            }
+
+            _context.Entry(kunde).State = EntityState.Modified;
+
+            try
+            {
+                await _context.SaveChangesAsync();
+            }
+            catch (DbUpdateConcurrencyException)
+            {
+                if (!KundeExists(id))
+                {
+                    return NotFound();
+                }
+                else
+                {
+                    throw;
+                }
+            }
+
+            return NoContent();
+        }
+
+        // POST: api/Kunden
+        // To protect from overposting attacks, see https://go.microsoft.com/fwlink/?linkid=2123754
+        [HttpPost]
+        public async Task<ActionResult<Kunde>> PostKunde(Kunde kunde)
+        {
+          if (_context.Kundes == null)
+          {
+              return Problem("Entity set 'WebApiContext.Kundes'  is null.");
+          }
+            _context.Kundes.Add(kunde);
+            try
+            {
+                await _context.SaveChangesAsync();
+            }
+            catch (DbUpdateException)
+            {
+                if (KundeExists(kunde.Idkunde))
+                {
+                    return Conflict();
+                }
+                else
+                {
+                    throw;
+                }
+            }
+
+            return CreatedAtAction("GetKunde", new { id = kunde.Idkunde }, kunde);
+        }
+
+        // DELETE: api/Kunden/5
+        [HttpDelete("{id}")]
+        public async Task<IActionResult> DeleteKunde(int id)
+        {
+            if (_context.Kundes == null)
+            {
+                return NotFound();
+            }
+            var kunde = await _context.Kundes.FindAsync(id);
+            if (kunde == null)
+            {
+                return NotFound();
+            }
+
+            _context.Kundes.Remove(kunde);
+            await _context.SaveChangesAsync();
+
+            return NoContent();
+        }
+
+        private bool KundeExists(int id)
+        {
+            return (_context.Kundes?.Any(e => e.Idkunde == id)).GetValueOrDefault();
+        }
+    }
+}