feat: lazy imports gapic v1 (#17673)

## Description
This PR initiates the rollout of our PEP 0810 lazy-loading architecture
to google-api-core, starting with the `gapic_v1` module.

By applying Python 3.15's native explicit lazy imports, we defer the
eager execution of inner modules (like `method.py` and `config.py`)
which recursively pull in massive third-party C-extensions like
`grpcio`. This acts as the first step in addressing the high
initialization latency and peak memory footprints we're seeing on
Serverless cold-starts.

Older Python runtimes (3.14 and below) safely ignore the
`__lazy_modules__` set and fall back to standard eager execution,
meaning this introduces zero backwards compatibility risk.

*(Note: We use fully-qualified, hardcoded module strings rather than
dynamic `__name__` injection, and provide explicit type annotations
(`Set[str]`), to ensure static analysis tools and type checkers can
safely infer the proxies without complaints).*

## Related Documents
* **Design Doc:** go/sdk:api-core-lazy-loading
* **Original GAPIC Generator Implementation PR:** #17591

Author: hebaalazzeh

packages/google-api-core/google/api_core/gapic_v1/__init__.py

@@ -12,18 +12,44 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from google.api_core.gapic_v1 import client_info
-from google.api_core.gapic_v1 import config
-from google.api_core.gapic_v1 import config_async
-from google.api_core.gapic_v1 import method
-from google.api_core.gapic_v1 import method_async
-from google.api_core.gapic_v1 import routing_header
+import importlib.util
+from typing import Set
 
-__all__ = [
-    "client_info",
-    "config",
-    "config_async",
-    "method",
-    "method_async",
-    "routing_header",
-]
+_has_grpc = importlib.util.find_spec("grpc") is not None
+
+# PEP 0810: Explicit Lazy Imports
+# Python 3.15+ natively intercepts and defers these imports.
+# Developers can disable this behavior and force eager imports.
+# For more information, see:
+# https://docs.python.org/3.15/library/sys.html#sys.set_lazy_imports_filter
+# Older Python versions safely ignore this variable.
+__lazy_modules__: Set[str] = {
+    "google.api_core.gapic_v1.client_info",
+    "google.api_core.gapic_v1.routing_header",
+}
+__all__ = ["client_info", "routing_header"]
+
+if _has_grpc:
+    __lazy_modules__.update(
+        {
+            "google.api_core.gapic_v1.config",
+            "google.api_core.gapic_v1.config_async",
+            "google.api_core.gapic_v1.method",
+            "google.api_core.gapic_v1.method_async",
+        }
+    )
+
+from google.api_core.gapic_v1 import (  # noqa: E402
+    client_info,
+    routing_header,
+)
+
+if _has_grpc:
+    from google.api_core.gapic_v1 import (  # noqa: F401
+        config,
+        config_async,
+        method,
+        method_async,
+    )
+
+    __all__.extend(["config", "config_async", "method", "method_async"])