Add local-dev opt-in for `ctx.http.fetch` to private/special-purpose addresses

[MichaelHindley]

Summary

When running SpacetimeDB locally, ctx.http.fetch rejects requests to private/special-purpose addresses (for example local
Docker services like MinIO on 127.0.0.1:9000 or 192.168.x.x), returning:

refusing to connect to private or special-purpose addresses

This blocks legitimate local-development setups where a module needs to call an object-storage service running on the same
machine/network, like in my case a S3 api compatible storage container.

Evidence

• Runtime log:
  • ProcedureHttpRequest returned errno: 21 (The HTTP request failed)
  • Error message: refusing to connect to private or special-purpose addresses
• Source references:
  • crates/core/src/host/instance_env.rs
    • BLOCKED_HTTP_ADDRESS_ERROR constant
    • FilteredDnsResolver filters resolved IPs via is_blocked_ip(...)
    • blocked ranges include loopback/private/link-local/etc

Reproduction

1. Run SpacetimeDB locally.
2. Create a procedure that calls ctx.http.fetch to a local service endpoint (for example http://127.0.0.1:9000/...).
3. Invoke the procedure.
4. Observe failure with the blocked-address error above.

Why this matters

This is a sensible default for production SSRF protection, but local development often relies on Docker-local services and
private network addresses. There is currently no documented local-dev override.

Requested behavior

Add an explicit opt-in local-dev override so private/special-purpose address egress can be enabled intentionally during
local development only.

Possible safe designs:

1. spacetime start flag (for local only), e.g. --allow-private-http-egress
2. Config-based allowlist of hostnames/CIDRs for module HTTP egress
3. Capability-style per-database/per-module egress policy

Thanks for considering this — this would unblock common local Docker workflows without changing production defaults.

[joshua-spacetime]

Definitely a reasonable ask. There is a feature flag allow_loopback_http_for_tests you can use if you're building from source, but we'll improve the ux for this.