Enable HTTP header customization for URLOperations

[mih]

this would be the patch:

diff --git a/datalad_next/url_operations/http.py b/datalad_next/url_operations/http.py
index 8700b7b..2d30c64 100644
--- a/datalad_next/url_operations/http.py
+++ b/datalad_next/url_operations/http.py
@@ -36,13 +36,24 @@ class HttpUrlOperations(UrlOperations):
     authentication challenges.
     """
 
-    _headers = {
-        'user-agent': user_agent('datalad', datalad.__version__),
-    }
+    def __init__(self, cfg=None, headers=None):
+        """
+        Parameters
+        ----------
+        cfg: ConfigManager, optional
+          A config manager instance that is consulted for any configuration
+          filesystem configuration individual handlers may support.
+        """
+        super().__init__(cfg=cfg)
+        self._headers = {
+            'user-agent': user_agent('datalad', datalad.__version__),
+        }
+        if headers:
+            self._headers.update(headers)
 
     def get_headers(self, headers: Dict | None = None) -> Dict:
         # start with the default
-        hdrs = dict(HttpUrlOperations._headers)
+        hdrs = dict(self._headers)
         if headers is not None:
             hdrs.update(headers)
         return hdrs

With the headers exposed in the constructor, we can use handler customization (that still needs to be documented #335) and supply custom headers like so:

datalad \
  -c datalad.url-handler.https://fz-juelich.sciebo.de/public.php/webdav.class=datalad_next.url_operations.http.HttpUrlOperations \
   -c 'datalad.url-handler.https://fz-juelich.sciebo.de/public.php/webdav.kwargs={"headers": {"X-Requested-With": "XMLHttpRequest"}}' \
   download \
   'https://fz-juelich.sciebo.de/public.php/webdav/king/hubert-neufeld-j-udI4zim2E-unsplash.jpg'