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 },
  }
  

Findings

Enable http2 in native fetch (via undici config)

http2 can be enabled by setting a flag on the underlying undici implementation

process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0'

setGlobalDispatcher(
  new Agent({
    allowH2: true,
  }),
)

Diagnostics for undici can also be enabled by subscribing to diagnostics channels:
https://github.com/nodejs/undici/blob/main/docs/docs/api/DiagnosticsChannel.md

This works in a basic nodejs application, but we were unable to get things fully working inside of vscode. There seems to be something different in the header objects used (possibly vscode swapping out Buffer or some other data structure) for something more compatible with its env). Regardless of the cause, there is an error with calling push on an undefined object that we couldn't work around.

Minimal example of test here:
https://github.com/bmingles/deephaven-repl/blob/5a9beb190410d942f2a6073b0deb597b43902d56/src/fetchtest.mts

Incorporating DH apis here:
https://github.com/bmingles/deephaven-repl/blob/5a9beb190410d942f2a6073b0deb597b43902d56/src/community.mts

node:http2

I was able to successfully make a connection (and see expected header responses) using node:http2. Colin and I are planning to implement a gRPC transport backed by node:http2 instead of relying on fetch. He will expose a way to provide this to the jsapi (deephaven/deephaven-core#6404), and I will implement the transport on the node side of things.