Add additional VertexAI Params (#5837)
## Changes
- Added the `stop` param to the `_VertexAICommon` class so it can be set
at llm initialization
## Example Usage
```python
VertexAI(
# ...
temperature=0.15,
max_output_tokens=128,
top_p=1,
top_k=40,
stop=["\n```"],
)
```
## Possible Reviewers
- @hwchase17
- @agola11
