Migrate Spanner Mock Server to Monorepo

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this feature request.

### Summary of the feature request

Migrate Spanner Mock Server to Monorepo

### Desired code experience

file: main.py
```python
def my_ideal_code():
...
some_data = my_new_feature()
do_something_with(some_data))
```

### Expected results

_e.g. `my_new_feature()` should return FOO_

### API client name and version

_No response_

### Use case

_e.g. This feature would be useful in a scenario where I need to process large amounts of data efficiently._

### Additional context

_e.g. This feature would help improve the performance of my application._