Add support for GCS deps

Also take out GCS calling logic from download_google_storage and
into call_google_storage.

GCS deps look like:
   'src/third_party/node/linux': {
       'dep_type': 'gcs',
       'condition': 'checkout_linux',
       'bucket': 'chromium-nodejs/20.11.0',
       'object_name': '46795170ff5df9831955f163f6966abde581c8af',
       'sha256sum': '887504c37404898ca41b896f448ee6d7fc24179d8fb6a4b79d028ab7e1b7153d',
   },

   'src/third_party/llvm-build/Release+Asserts': {
       'dep_type': 'gcs',
       'condition': 'checkout_linux',
       'bucket': 'chromium-browser-clang',
       'object_name': 'Linux_x64/clang-llvmorg-18-init-17730-gf670112a-2.tar.xz',
       'sha256sum': '1e46df9b4e63c074064d75646310cb76be2f19815997a8486987189d80f991e8',
   },

Example directory for src/third_party/node/linux after gclient sync:
- tar_file.gz is the downloaded file from GCS.
- node_linux_x64/ is extracted in its path.
- `hash` contains the sha of GCS filename.
```
chromium/src/ ->
   third_party/node/linux/ ->
       hash, tar_file.gz, node_linux_x64/
```

Bug: b/324418194
Change-Id: Ibcbbff27e211f194ddb8a08494af56570a84a12b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5299722
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Reviewed-by: Joanna Wang <jojwang@chromium.org>

gclient_eval.py

@@ -131,6 +131,14 @@ _GCLIENT_DEPS_SCHEMA = _NodeDictSchema({
             schema.Optional('dep_type', default='cipd'):
             str,
         }),
+        # GCS content.
+        _NodeDictSchema({
+            'bucket': str,
+            'object_name': str,
+            'sha256sum': str,
+            schema.Optional('condition'): str,
+            schema.Optional('dep_type', default='gcs'): str,
+        }),
     ),
 })