xlang-go: fetch from `zipURL` when present in `initializationOptions`

Created by: chrismwendt

This adds support to xlang-go for checking for a new zipURL field in the initializationOptions in the initialize LSP request, and fetching the current repo's files from there if it's present. xlang-go will fall back to the old behavior of fetching from gitserver when that field is absent.

zipURL will be constructed by lang-go and passed to Go buildserver through LSP Proxy (see the commit where I added it to lang-go).

This PR does not need to update the CHANGELOG because