feat(core): auto detect proxy server on load (#3195)

* feat: auto detect proxy server on load

* fix: opt-in for auto proxy server detection

packages/netlify-cms-core/src/constants/__tests__/configSchema.spec.js

@@ -126,5 +126,31 @@ describe('config', () => {
         });
       }).toThrowError("'collections[0]' should be object");
     });
+
+    it('should throw if local_backend is not a boolean or plain object', () => {
+      expect(() => {
+        validateConfig(merge(validConfig, { local_backend: [] }));
+      }).toThrowError("'local_backend' should be boolean");
+    });
+
+    it('should throw if local_backend is a plain object but missing url property', () => {
+      expect(() => {
+        validateConfig(merge(validConfig, { local_backend: {} }));
+      }).toThrowError("'local_backend' should be object");
+    });
+
+    it('should not throw if local_backend is a boolean', () => {
+      expect(() => {
+        validateConfig(merge(validConfig, { local_backend: true }));
+      }).not.toThrowError();
+    });
+
+    it('should not throw if local_backend is a plain object with url property', () => {
+      expect(() => {
+        validateConfig(
+          merge(validConfig, { local_backend: { url: 'http://localhost:8081/api/v1' } }),
+        );
+      }).not.toThrowError();
+    });
   });
 });

packages/netlify-cms-core/src/constants/configSchema.js

@@ -42,6 +42,18 @@ const getConfigSchema = () => ({
       },
       required: ['name'],
     },
+    local_backend: {
+      oneOf: [
+        { type: 'boolean' },
+        {
+          type: 'object',
+          properties: {
+            url: { type: 'string', examples: ['http://localhost:8081/api/v1'] },
+          },
+          required: ['url'],
+        },
+      ],
+    },
     locale: { type: 'string', examples: ['en', 'fr', 'de'] },
     site_url: { type: 'string', examples: ['https://example.com'] },
     display_url: { type: 'string', examples: ['https://example.com'] },