Access client IP from SDK

[ecumene]

We can already get the IP when using WAGI through the REMOTE_ADDR environment variable, but there's no way to access the client's IP when using the SDK.

Maybe we could use this standard way that proxies use to forward requests

[itowlson]

@radu-matei Could/should we add it to the request record in http-types.wit, or is that set in stone?

[radu-matei]

Updating the request object is a breaking change — we should seriously consider it before we approach stabilization, and this might be suitable for adding.

https://github.com/fermyon/spin/blob/c0f2b3fd6ff5751772f9f90bbb25d017475f6e6e/crates/http/src/lib.rs#L351-L361

In the meantime, we could consider adding the information in the way @ecumene suggested — https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For