add -forceCache flag to override no-store and private directives

The httpcache package is intended only to be used in private caches, so
it will cache responses marked `private` like normal.  However,
imageproxy is a shared cache, so these response should not be cached
under normal circumstances.  This change introduces a potentially
breaking change to start respecting the `private` cache directive in
responses.

This also adds a new `-forceCache` flag to ignore the `private` and
`no-store` directives, and cache all responses regardless.

imageproxy_test.go

@@ -377,6 +377,7 @@ func TestProxy_UpdateCacheHeaders(t *testing.T) {
 	tests := []struct {
 		name        string
 		minDuration time.Duration
+		forceCache  bool
 		headers     http.Header
 		want        http.Header
 	}{
@@ -398,6 +399,14 @@ func TestProxy_UpdateCacheHeaders(t *testing.T) {
 				"Cache-Control": {"max-age=600"},
 			},
 		},
+		{
+			name:        "min duration, no header",
+			minDuration: 30 * time.Second,
+			headers:     http.Header{},
+			want: http.Header{
+				"Cache-Control": {"max-age=30"},
+			},
+		},
 		{
 			name:        "cache control exceeds min duration",
 			minDuration: 30 * time.Second,
@@ -457,11 +466,53 @@ func TestProxy_UpdateCacheHeaders(t *testing.T) {
 				"Cache-Control": {"max-age=3600"},
 			},
 		},
+		{
+			name: "respect no-store",
+			headers: http.Header{
+				"Cache-Control": {"max-age=600, no-store"},
+			},
+			want: http.Header{
+				"Cache-Control": {"max-age=600, no-store"},
+			},
+		},
+		{
+			name: "respect private",
+			headers: http.Header{
+				"Cache-Control": {"max-age=600, private"},
+			},
+			want: http.Header{
+				"Cache-Control": {"max-age=600, no-store, private"},
+			},
+		},
+		{
+			name:       "force cache, normalize directives",
+			forceCache: true,
+			headers: http.Header{
+				"Cache-Control": {"MAX-AGE=600, no-store, private"},
+			},
+			want: http.Header{
+				"Cache-Control": {"max-age=600"},
+			},
+		},
+		{
+			name:        "force cache with min duration",
+			minDuration: 1 * time.Hour,
+			forceCache:  true,
+			headers: http.Header{
+				"Cache-Control": {"max-age=600, private, no-store"},
+			},
+			want: http.Header{
+				"Cache-Control": {"max-age=3600"},
+			},
+		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			p := &Proxy{MinimumCacheDuration: tt.minDuration}
+			p := &Proxy{
+				MinimumCacheDuration: tt.minDuration,
+				ForceCache:           tt.forceCache,
+			}
 			hdr := maps.Clone(tt.headers)
 			p.updateCacheHeaders(hdr)