ci: add skip directives for long tests (#3151)

This PR introduces many CI optimizations:

1. The `[ci-skip]` PR body directive to skip the Postgres and end to end tests
2. Improved caching that cuts the Go test matrix in half
3. Increasing Go test parallelism for ~20% gains
4. Enable caching in webpack (4x frontend build)

.github/workflows/prcontext/parse_test.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func Test_parseBody_basic(t *testing.T) {
+	parseBody(`
+This is a test PR.	
+
+[ci-skip postgres windows]
+	`)
+}
+
+func Test_parseBody(t *testing.T) {
+	type args struct {
+		body string
+	}
+	tests := []struct {
+		name      string
+		args      args
+		wantSkips []string
+	}{
+		{"no directive", args{"test pr 123\n\n"}, nil},
+		{"single dir single skip", args{"test pr [ci-skip dog] 123\n\n"}, []string{"dog"}},
+		{"double dir double skip", args{"test pr [ci-skip dog] [ci-skip cat] 123\n\n"}, []string{"dog", "cat"}},
+		{"single dir double skip", args{"test pr [ci-skip test/go/postgres cat] 123\n\n"}, []string{"test/go/postgres", "cat"}},
+		{"confuse", args{"ci ci [ci-skip] dog [ci-skip test/go/postgres test/e2e/ubuntu-latest] 123\n\n"}, []string{"test/go/postgres", "test/e2e/ubuntu-latest"}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if gotSkips := parseBody(tt.args.body); !reflect.DeepEqual(gotSkips, tt.wantSkips) {
+				t.Errorf("parseBody() = %v, want %v", gotSkips, tt.wantSkips)
+			}
+		})
+	}
+}