[Fix] Fix DatabricksConfig.copy when authenticated with OAuth (#723)

## Changes
<!-- Summary of your changes that are easy to understand -->
`DatabricksCliTokenSource().token()` itself can't be copied. So, Deep
Copy can't be performed for Config.
Added the wrapper function which can be copied. So, Deep copy can be
performed.
## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [ ] `make test` run locally
- [ ] `make fmt` applied
- [ ] relevant integration tests applied

databricks/sdk/credentials_provider.py

@@ -607,7 +607,10 @@ def inner() -> Dict[str, str]:
         token = token_source.token()
         return {'Authorization': f'{token.token_type} {token.access_token}'}
 
-    return OAuthCredentialsProvider(inner, token_source.token)
+    def token() -> Token:
+        return token_source.token()
+
+    return OAuthCredentialsProvider(inner, token)
 
 
 class MetadataServiceTokenSource(Refreshable):

tests/test_config.py

@@ -1,10 +1,15 @@
 import os
+import pathlib
 import platform
+import random
+import string
+from datetime import datetime
 
 import pytest
 
 from databricks.sdk import useragent
 from databricks.sdk.config import Config, with_product, with_user_agent_extra
+from databricks.sdk.credentials_provider import Token
 from databricks.sdk.version import __version__
 
 from .conftest import noop_credentials, set_az_path
@@ -79,6 +84,40 @@ def test_config_copy_deep_copies_user_agent_other_info(config):
     useragent._reset_extra(original_extra)
 
 
+def test_config_deep_copy(monkeypatch, mocker, tmp_path):
+    mocker.patch('databricks.sdk.credentials_provider.CliTokenSource.refresh',
+                 return_value=Token(access_token='token',
+                                    token_type='Bearer',
+                                    expiry=datetime(2023, 5, 22, 0, 0, 0)))
+
+    write_large_dummy_executable(tmp_path)
+    monkeypatch.setenv('PATH', tmp_path.as_posix())
+
+    config = Config(host="https://abc123.azuredatabricks.net", auth_type="databricks-cli")
+    config_copy = config.deep_copy()
+    assert config_copy.host == config.host
+
+
+def write_large_dummy_executable(path: pathlib.Path):
+    cli = path.joinpath('databricks')
+
+    # Generate a long random string to inflate the file size.
+    random_string = ''.join(random.choice(string.ascii_letters) for i in range(1024 * 1024))
+    cli.write_text("""#!/bin/sh
+cat <<EOF
+{
+"access_token": "...",
+"token_type": "Bearer",
+"expiry": "2023-05-22T00:00:00.000000+00:00"
+}
+EOF
+exit 0
+""" + random_string)
+    cli.chmod(0o755)
+    assert cli.stat().st_size >= (1024 * 1024)
+    return cli
+
+
 def test_load_azure_tenant_id_404(requests_mock, monkeypatch):
     set_az_path(monkeypatch)
     mock = requests_mock.get('https://abc123.azuredatabricks.net/aad/auth', status_code=404)