feat(host): add HostServices for guest calls back to the host

[DamianReeves]

Part of finos/morphir#958, phase D (finos/morphir#962). Split out of #237, which closed with the rest of the session layer in #275.

What

Let a guest call back to its host for logging, progress and reading files. No JSON-RPC guest calls the host today, so this lands with the component imports in phase D.

• Channel::receive carries guest requests and notifications as well as responses.
• JsonRpcConnection runs a dispatch loop: while it waits for a response, it hands guest requests and notifications to the host.
• A client-supplied HostServices trait in morphir-host (log, progress, read_file), portable to wasm32 like the rest of the crate.
• The Extism host functions in morphir-host-native (morphir_log and the workspace and cache functions) are bound to the same trait instead of today's stubs.

Done when

• A guest notification or request reaches a HostServices implementation through JsonRpcConnection, pinned by a fake-channel test.
• An Extism guest's host-function call reaches the same trait.
• morphir-host still checks on wasm32-unknown-unknown.

Design

Host library spec (beads morphir-opua.1, design), section on HostServices. Tracked in beads: morphir-opua.1.4.5.