DHE: Envoy Support

[bmingles]

We need to add envoy support to the vscode extension:

• In a non-envoy configuration, the grpcUrl reflects the worker url + port (currently supported, but we are currently using this to determine where the jsapi is rooted from)
• In an envoy configuration, the grpcUrl will be the same as the server, but we use an "envoy header" to make things work. The jsapi for a worker will be at /worker/[someenvoyprefix]/jsapi/dh-core.js (this is provided via designated.jsApiUrl prop on the query info for the worker)

Things to implement:

• The DhcService needs to be refactored to account for the 2 urls
• Can probably use the jsApiUrl as the source of truth for where to dowload the jsapi. Will need to account for the fact that it is the full URL since current logic appends jsapi/dh-core.js to the grpcUrl
• Creating the Core+ client will need to include the an envoy prefix header in the options:

  {
     headers: { 'envoy-prefix': envoyPrefix },
  }