feat: microgen - adds client.py.j2 template (#2298)

- [x] Follows: PR #2297  (should be merged after that PR is merged.)

This PR adds a `client.py.j2` template to enable creation of the `client.py` file that contains the `BigQueryClient` class.

Author: chalmerlowe

scripts/microgenerator/templates/client.py.j2

@@ -0,0 +1,122 @@
+# -*- coding: utf-8 -*-
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# Imports
+import os
+
+from typing import (
+    Any,
+    Dict,
+    List,
+    Optional,
+    Sequence,
+    Tuple,
+    Union,
+)
+
+# Service Client Imports
+{% for imp in service_imports %}{{ imp }}{% endfor %}
+
+# Helper Imports
+from . import _helpers
+
+# Type Imports
+{% for imp in type_imports %}{{ imp }}{% endfor %}
+
+from google.api_core import client_options as client_options_lib
+from google.api_core import gapic_v1
+from google.api_core import retry as retries
+from google.auth import credentials as auth_credentials
+import google.auth
+
+# Create type aliases
+try:
+    OptionalRetry = Union[retries.Retry, gapic_v1.method._MethodDefault, None]
+except AttributeError:  # pragma: NO COVER
+    OptionalRetry = Union[retries.Retry, object, None]  # type: ignore
+
+DEFAULT_RETRY: OptionalRetry = gapic_v1.method.DEFAULT
+DEFAULT_TIMEOUT: Union[float, object] = gapic_v1.method.DEFAULT
+DEFAULT_METADATA: Sequence[Tuple[str, Union[str, bytes]]] = ()
+
+{#- Create a mapping from the ServiceClient class name to its property name on the main client.
+    e.g., {'DatasetServiceClient': 'dataset_service_client'}
+-#}
+{% set class_to_instance_map = {} %}
+{% for service in services %}
+    {% set _ = class_to_instance_map.update({service.service_client_class: service.property_name}) %}
+{% endfor %}
+
+
+class BigQueryClient:
+    def __init__(
+        self,
+        project: Optional[str] = None,
+        credentials: Optional[auth_credentials.Credentials] = None,
+        client_options: Optional[client_options_lib.ClientOptions] = None,
+    ):
+        if credentials is None:
+            credentials, project_id = google.auth.default()
+        else:
+            project_id = None  # project_id is not available from non-default credentials
+
+        if project is None:
+            project = project_id
+
+        self.project = project
+        self._credentials = credentials
+        self._client_options = client_options
+        self._clients: Dict[str, Any] = {}
+
+    # --- *METHOD SECTION ---
+
+{% for method in methods %}
+{% if method.request_id_args is not none and method.request_id_args|length > 0 %}
+{% filter indent(4, True) %}
+    {% include 'partials/_method_with_request_builder.j2' %}
+{% endfilter %}
+{% else %}
+{% filter indent(4, True) %}
+    {% include 'partials/_simple_passthrough_method.j2' %}
+{% endfilter %}
+{% endif %}
+
+
+{% endfor %}
+
+{#- *ServiceClient Properties Section: methods to get/set service clients -#}
+    # --- *SERVICECLIENT PROPERTIES ---
+{% for service in services %}
+    @property
+    def {{ service.property_name }}(self):
+        if "{{ service.service_name }}" not in self._clients:
+            self._clients["{{ service.service_name }}"] = {{ service.service_module_name }}.{{ service.service_client_class }}(
+                credentials=self._credentials, client_options=self._client_options
+            )
+        return self._clients["{{ service.service_name }}"]
+
+    @{{ service.property_name }}.setter
+    def {{ service.property_name }}(self, value):
+        if not isinstance(value, {{ service.service_module_name }}.{{ service.service_client_class }}):
+            raise TypeError(
+                "Expected an instance of {{ service.service_client_class }}."
+            )
+        self._clients["{{ service.service_name }}"] = value
+
+{% endfor %}
+
+{#- Helper Section: methods included from partial template -#}
+    {#- include "partials/_client_helpers.j2" #}