Draft: Support multiple APIs

This MR implements Support new gwdatafind server APIs (#28) by refactoring gwdatafind.api into a subpackage, and implementing a decorator for the gwdatafind.ui methods that selects the xxx_path function from the right API module dynamically.

Also adds a new -A/--api option for the command-line interface.

gwdatafind/api/__init__.py

+# Copyright (C) 2025 Cardiff University
+#
+# This file is part of GWDataFind.
+#
+# GWDataFind is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# GWDataFind is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GWDataFind.  If not, see <http://www.gnu.org/licenses/>.
+
+"""API definitions for the GWDataFind Client."""
+
+# list of APIs
+APIS = (
+    "ldr",
+)