Enable common folder ouside of adk-agent for code reusability

### Is your feature request related to a specific problem?
I have a couple of agents that are in a repo. I can run them via `adk web` and can deploy them via `adk deploy agent_engine`, however, I cannot share code between them. This would be really handy for reusing tools and skills in multiple similar agents in the same repo.

### Describe the Solution You'd Like
Best scenario, the `adk deploy` command can validate references and can get all the necessary extra files/folders.
Ok scenario: a predefined `common` folder that can hold common code automatically picked up during deployment.

### Impact on your work
How does this feature impact your work and what are you trying to achieve?
Code reuse: no need to copy-paste the same code across multiple folders.
Code sync keeps the same files in sync.

### Willingness to contribute
Are you interested in implementing this feature yourself or submitting a PR?
Yes, with some guidance, glad to help out.
I also looked at the code and for agent engine there is a line on 1130 in `cli_deploy.py` where the extra folders could be added with a cli option --extra_source_packages
```python
agent_config['source_packages'] = [temp_folder] + **extra_source_packages**
```
For the Docker version, I think it is even simpler because we would just copy the common folder alongside the agent.