Passthrough proxying on xrpc-server

[devinivy]

This implements http passthroughs on xrpc-server and a proxy handler for forwarding the active request to another service. It's also now used on the relevant proxied app.bsky routes on the PDS to the appview. Some notes:

• Can return { passthru: http.IncomingRequest } from xrpc-server handlers, and the request will be faithfully passed through.
  • Updated server codegen to reflect this.
• Implemented xrpcServer.proxy() which proxies the active request using a passthrough.
• Fixes a header normalization issue in the xrpc client. One "soft" breaking change is that defaultFetchHandler() is no longer responsible for header normalization, and assumes the headers passed to it have already been normalized to lower-case.
• Where possible PDS routes for app.bsky are now proxied using xrpcServer.proxy(). The exceptions are generally read-after-write endpoints.
  • These routes will now start passing through response headers from appview.

[dholms]

this is beautiful 😍 🙇‍♂️

[devinivy]

Closing this but keeping the branch around—will probably redo this in the future and hit all proxying services.