wasi-http: Allow setting TLS root certs for `default_send_request`

Currently, the TLS roots are [hard-coded](https://github.com/bytecodealliance/wasmtime/blob/ca405bb023d7f5f201eed8cdf37be3a9f3141e5b/crates/wasi-http/src/types.rs#L232-L239) to the `webpki-roots` set. This is a good default, but in some scenarios private roots are required. We should be able to add options to `OutgoingRequestConfig` to extend and/or replace that default set of roots with custom root(s).

[Zulip context](https://bytecodealliance.zulipchat.com/#narrow/stream/219900-wasi/topic/HTTP.20client.20PKI.20roots)