Configurable cache path for List token (local tokenizer file path)

**Is your feature request related to a problem? Please describe.**
Currently Google aiplatform loads tokenizer by either downloading from github or reading from a tmpdir cache path which is not configurable at a library level. https://github.com/googleapis/python-aiplatform/blob/main/vertexai/tokenization/_tokenizer_loading.py#L136-L147

Can we make it configurable like how TikToken or NLK does it ? https://github.com/openai/tiktoken/blob/main/tiktoken/load.py#L34-L42 With a env variable like `VERTEX_TOKENIZER_CACHE_DIR` ?

**Describe the solution you'd like**

Our org does not allow network download of file on our deployment servers, so we need to uplaod the file to a fixed read only directory on the server. Being able to configure the path for that server would be useful.

**Describe alternatives you've considered**

I tried setting the TMPDIR env variable, but that works at python global level for all libraries and does not seem very configurable.