Support for proxies please!

**Is your feature request related to a problem? Please describe.**
The library does not work behind a proxy. This is because it uses node's in-built `fetch` function to post to Vertex, and `fetch` has no proxy support.

**Describe the solution you'd like**
Please use something other than `fetch` to call the Vertex and allow for proxy support in some way, such as a proxy config object being passed in to the VertexAI constructor (I have not through through the most appropriate way to configure the proxy settings). Note that GCP CLI supports this, so it seems odd that this library does not. It's critical to our use case so I will need to fork the SDK to be able to use it.

**Describe alternatives you've considered**
There are no alternatives. We can't use the SDK without this feature.