feat(s3 auth): add authorization support for s3 files provider (#3265)

# What does this PR do?

adds support for authorized users to the s3 files provider

## Test Plan

existing and new unit tests

tests/unit/providers/files/conftest.py

@@ -0,0 +1,62 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import boto3
+import pytest
+from moto import mock_aws
+
+from llama_stack.providers.remote.files.s3 import S3FilesImplConfig, get_adapter_impl
+from llama_stack.providers.utils.sqlstore.sqlstore import SqliteSqlStoreConfig
+
+
+class MockUploadFile:
+    def __init__(self, content: bytes, filename: str, content_type: str = "text/plain"):
+        self.content = content
+        self.filename = filename
+        self.content_type = content_type
+
+    async def read(self):
+        return self.content
+
+
+@pytest.fixture
+def sample_text_file():
+    content = b"Hello, this is a test file for the S3 Files API!"
+    return MockUploadFile(content, "sample_text_file-0.txt")
+
+
+@pytest.fixture
+def sample_text_file2():
+    content = b"Hello, this is a second test file for the S3 Files API!"
+    return MockUploadFile(content, "sample_text_file-1.txt")
+
+
+@pytest.fixture
+def s3_config(tmp_path):
+    db_path = tmp_path / "s3_files_metadata.db"
+
+    return S3FilesImplConfig(
+        bucket_name=f"test-bucket-{tmp_path.name}",
+        region="not-a-region",
+        auto_create_bucket=True,
+        metadata_store=SqliteSqlStoreConfig(db_path=db_path.as_posix()),
+    )
+
+
+@pytest.fixture
+def s3_client():
+    # we use `with mock_aws()` because @mock_aws decorator does not support
+    # being a generator
+    with mock_aws():
+        # must yield or the mock will be reset before it is used
+        yield boto3.client("s3")
+
+
+@pytest.fixture
+async def s3_provider(s3_config, s3_client):  # s3_client provides the moto mock, don't remove it
+    provider = await get_adapter_impl(s3_config, {})
+    yield provider
+    await provider.shutdown()