chore: Remove h2mux code

Some more legacy h2mux code to be cleaned up and moved out of the way.
The h2mux.Header used in the serialization for http2 proxied headers is moved to connection module. Additionally, the booleanfuse structure is also moved to supervisor as it is also needed. Both of these structures could be evaluated later for removal/updates, however, the intent of the proposed changes here is to remove the dependencies on the h2mux code and removal.

Approved-by: Chung-Ting Huang <chungting@cloudflare.com>
Approved-by: Luis Neto <lneto@cloudflare.com>
Approved-by: Gonçalo Garcia <ggarcia@cloudflare.com>

MR: https://gitlab.cfdata.org/cloudflare/tun/cloudflared/-/merge_requests/1576

supervisor/fuse.go

@@ -0,0 +1,50 @@
+package supervisor
+
+import "sync"
+
+// booleanFuse is a data structure that can be set once to a particular value using Fuse(value).
+// Subsequent calls to Fuse() will have no effect.
+type booleanFuse struct {
+	value int32
+	mu    sync.Mutex
+	cond  *sync.Cond
+}
+
+func newBooleanFuse() *booleanFuse {
+	f := &booleanFuse{}
+	f.cond = sync.NewCond(&f.mu)
+	return f
+}
+
+// Value gets the value
+func (f *booleanFuse) Value() bool {
+	// 0: unset
+	// 1: set true
+	// 2: set false
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.value == 1
+}
+
+func (f *booleanFuse) Fuse(result bool) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	newValue := int32(2)
+	if result {
+		newValue = 1
+	}
+	if f.value == 0 {
+		f.value = newValue
+		f.cond.Broadcast()
+	}
+}
+
+// Await blocks until Fuse has been called at least once.
+func (f *booleanFuse) Await() bool {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	for f.value == 0 {
+		f.cond.Wait()
+	}
+	return f.value == 1
+}