feat(checker): port other checkers over

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

lib/checker/headerexists/factory.go

@@ -0,0 +1,40 @@
+package headerexists
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+
+	"github.com/TecharoHQ/anubis/lib/checker"
+)
+
+type Factory struct{}
+
+func (f Factory) Build(ctx context.Context, data json.RawMessage) (checker.Interface, error) {
+	var headerName string
+
+	if err := json.Unmarshal([]byte(data), &headerName); err != nil {
+		return nil, fmt.Errorf("%w: want string", checker.ErrUnparseableConfig)
+	}
+
+	if err := f.Valid(ctx, data); err != nil {
+		return nil, err
+	}
+
+	return New(http.CanonicalHeaderKey(headerName)), nil
+}
+
+func (Factory) Valid(ctx context.Context, data json.RawMessage) error {
+	var headerName string
+
+	if err := json.Unmarshal([]byte(data), &headerName); err != nil {
+		return fmt.Errorf("%w: want string", checker.ErrUnparseableConfig)
+	}
+
+	if headerName == "" {
+		return fmt.Errorf("%w: string must not be empty", checker.ErrInvalidConfig)
+	}
+
+	return nil
+}