feat(checker): port other checkers over

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/checker/headerexists/checker.go

@@ -0,0 +1,32 @@
+package headerexists
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/TecharoHQ/anubis/internal"
+	"github.com/TecharoHQ/anubis/lib/checker"
+)
+
+func New(key string) checker.Interface {
+	return headerExistsChecker{
+		header: strings.TrimSpace(http.CanonicalHeaderKey(key)),
+		hash:   internal.FastHash(key),
+	}
+}
+
+type headerExistsChecker struct {
+	header, hash string
+}
+
+func (hec headerExistsChecker) Check(r *http.Request) (bool, error) {
+	if r.Header.Get(hec.header) != "" {
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func (hec headerExistsChecker) Hash() string {
+	return hec.hash
+}