feat: Add experimental rewrite of go2rtc live provider (MSE/WebRTC/MP4/MJPEG) (#2580)

- Closes #2556
- Closes #2450

**Key intended features:**
 - go2rtc compatible
- 100% test coverage to significantly improve ability to test, maintain
and work around browser weirdnesses (e.g. Safari).
 - Written from the ground up in the style of the rest of the project.

**To use:**
 - Change `live_provider` from `go2rtc` to `go2rtc-experimental`.

tests/utils/concurrency/generation.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from 'vitest';
+
+import { Generation } from '../../../src/utils/concurrency/generation';
+
+describe('Generation', () => {
+  it('should treat a fresh snapshot as current', () => {
+    const generation = new Generation();
+
+    expect(generation.isCurrent(generation.current())).toBe(true);
+  });
+
+  it('should keep a snapshot current across further snapshots', () => {
+    const generation = new Generation();
+    const token = generation.current();
+
+    generation.current();
+
+    expect(generation.isCurrent(token)).toBe(true);
+  });
+
+  it('should invalidate outstanding snapshots', () => {
+    const generation = new Generation();
+    const token = generation.current();
+
+    generation.invalidate();
+
+    expect(generation.isCurrent(token)).toBe(false);
+  });
+
+  it('should invalidate prior tokens when starting a new operation', () => {
+    const generation = new Generation();
+    const first = generation.next();
+    const second = generation.next();
+
+    expect(generation.isCurrent(first)).toBe(false);
+    expect(generation.isCurrent(second)).toBe(true);
+  });
+});