Make WorkspaceClient and AccountClient more friendly with autospeccing (#480)

## Changes
We cannot `create_autospec(WorkspaceClient)`, because all public
properties are created within the constructor:

```
>>> WorkspaceClient
<class 'databricks.sdk.WorkspaceClient'>
>>> ws = create_autospec(WorkspaceClient)
>>> ws
<MagicMock spec='WorkspaceClient' id='4394200816'>
>>> ws.workspace.list()
Traceback (most recent call last):
  File "/Applications/PyCharm CE.app/Contents/plugins/python-ce/helpers/pydev/_pydevd_bundle/pydevd_exec2.py", line 3, in Exec
    exec(exp, global_vars, local_vars)
  File "<input>", line 1, in <module>
  File "/opt/homebrew/Cellar/[email protected]/3.10.12_1/Frameworks/Python.framework/Versions/3.10/lib/python3.10/unittest/mock.py", line 643, in __getattr__
    raise AttributeError("Mock object has no attribute %r" % name)
AttributeError: Mock object has no attribute 'workspace'
```

This PR makes it easier to introspect the class instances and makes
mocks more reliable.

All existing behavior stays the same.

See:
- https://docs.python.org/3/library/unittest.mock.html#autospeccing
- https://stackoverflow.com/a/35921144/277035


## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

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

Author: nfx

.codegen/__init__.py.tmpl

@@ -52,14 +52,34 @@ class WorkspaceClient:
             debug_headers=debug_headers,
             product=product,
             product_version=product_version)
-        self.config = config.copy()
-        self.dbutils = _make_dbutils(self.config)
-        self.api_client = client.ApiClient(self.config)
+        self._config = config.copy()
+        self._dbutils = _make_dbutils(self._config)
+        self._api_client = client.ApiClient(self._config)
 
         {{- range .Services}}{{if not .IsAccounts}}
-        self.{{.SnakeName}}: {{template "api" .}} = {{template "api" .}}(self.api_client){{if .Description}}
-        """{{.Comment "        " 100}}"""{{end}}
-        {{end -}}{{end}}
+        self._{{.SnakeName}} = {{template "api" .}}(self._api_client){{end -}}{{end}}
+
+    @property
+    def config(self) -> client.Config:
+        return self._config
+
+    @property
+    def api_client(self) -> client.ApiClient:
+        return self._api_client
+
+    @property
+    def dbutils(self) -> dbutils.RemoteDbUtils:
+        return self._dbutils
+
+    {{- range .Services}}{{if not .IsAccounts}}
+    @property
+    def {{.SnakeName}}(self) -> {{template "api" .}}:
+        {{if .Description}}"""{{.Summary}}"""{{end}}
+        return self._{{.SnakeName}}
+    {{end -}}{{end}}
+
+    def __repr__(self):
+        return f"WorkspaceClient(host='{self._config.host}', auth_type='{self._config.auth_type}', ...)"
 
 class AccountClient:
     """
@@ -80,10 +100,26 @@ class AccountClient:
             debug_headers=debug_headers,
             product=product,
             product_version=product_version)
-        self.config = config.copy()
-        self.api_client = client.ApiClient(self.config)
+        self._config = config.copy()
+        self._api_client = client.ApiClient(self._config)
 
         {{- range .Services}}{{if .IsAccounts}}
-        self.{{(.TrimPrefix "account").SnakeName}}: {{template "api" .}} = {{template "api" .}}(self.api_client){{if .Description}}
-        """{{.Comment "        " 100}}"""{{end}}
-        {{end -}}{{end}}
+        self._{{(.TrimPrefix "account").SnakeName}} = {{template "api" .}}(self._api_client){{end -}}{{end}}
+
+    @property
+    def config(self) -> client.Config:
+        return self._config
+
+    @property
+    def api_client(self) -> client.ApiClient:
+        return self._api_client
+
+    {{- range .Services}}{{if .IsAccounts}}
+    @property
+    def {{(.TrimPrefix "account").SnakeName}}(self) -> {{template "api" .}}:{{if .Description}}
+        """{{.Summary}}"""{{end}}
+        return self._{{(.TrimPrefix "account").SnakeName}}
+    {{end -}}{{end}}
+
+    def __repr__(self):
+        return f"AccountClient(account_id='{self._config.account_id}', auth_type='{self._config.auth_type}', ...)"