Add basic outline of a Fido client

Doesn't do anything yet until I know how to query SDC's database.

python-user-api/client.py

+import sunpy.net.attrs as a
+from sunpy.net.attr import AttrAnd, AttrOr, AttrWalker, DataAttr
+from sunpy.net.base_client import BaseClient, QueryResponseTable
+
+
+walker = AttrWalker()
+
+
+@walker.add_creator(AttrOr)
+def create_or(wlk, tree):
+    """"""
+    results = []
+    for sub in tree.attrs:
+        results.append(wlk.create(sub))
+
+    return results
+
+
+@walker.add_creator(AttrAnd, DataAttr)
+def create_and(wlk, tree):
+    """"""
+    result = dict()
+    wlk.apply(tree, result)
+    return [result]
+
+
+@walker.add_applier(a.Time)
+def _(wlk, attr, params):
+    """"""
+    return params.update({"startTime": attr.start.isot, "endTime": attr.end.isot})
+
+
+@walker.add_applier(a.Level)
+def _(wlk, attr, params):
+    """"""
+    return params.update({"level": attr.value})
+
+
+class KISClient(BaseClient):
+    def __init__(self):
+        """"""
+
+    def _query_api(self, query_params):
+        """API query process TBD."""
+
+    def search(self, query):
+        """"""
+        queries = walker.create(query)
+
+        results = []
+        for query_params in queries:
+            results.append(self._query_api(query_params))
+
+        return QueryResponseTable(results, client=self)
+
+    def fetch(self):
+        """"""
+
+    @classmethod
+    def _can_handle_query(cls, *query):
+        """"""
+        query_attrs = set(type(x) for x in query)
+        supported_attrs = {a.Time, a.Level}
+        return supported_attrs.issuperset(query_attrs)
+
+    @classmethod
+    def register_values(cls):
+        """"""
+        adict = {
+            a.Provider: [
+                (
+                    "KIS-SDC",
+                    "Science Data Centre at the Leibniz-Institut für Sonnenphysik",
+                )
+            ]
+        }
+
+        return adict