Improve Apps Script relay and example configs

[poulcarlsen53]

Summary

This PR improves the Apps Script forwarder and example configs without changing the tunnel protocol.

Changes:

• Adds RELAY_URLS support for multiple VPS tunnel endpoints.
• Adds a relay loop guard so Apps Script cannot accidentally relay to another Apps Script URL.
• Makes invocation counting optional to avoid a PropertiesService write on every tunnel request.
• Makes example configs safer to copy by avoiding invalid placeholder values.

Verification

• go test -count=1 ./...
• go vet ./...

Compatibility

The wire protocol is unchanged. Existing client/server deployments continue to work after redeploying Code.gs.

[Kianmhz]

Thanks for this. The Apps Script changes here are solid and I'd like to land them. Before merging, I'd like to trim the config-example pieces, because most of the new fields they introduce aren't read by internal/config yet. They belong with the larger perf bundle in #131. Right now encoding/json silently drops them, so users copying the example would think they're configuring something when they're not.

Keep as-is:

• All of apps_script/Code.gs: RELAY_URLS, the GAS_RELAY_LOOP_RE guard, and ENABLE_INVOCATION_COUNTING are all wins, no Go-side dependency.
• In server_config.example.json: the upstream_proxy: "" fix and the _comment_upstream_proxy note. The old "OPTIONAL_socks5://…" placeholder fails to parse if copied verbatim, good catch.
• In client_config.example.json: trimming script_keys to a single entry plus the _comment_script_keys note for adding more.

Please drop from this PR (move to #131 alongside the code that reads them):

• client_config.example.json: transport_mode, direct_stream_urls, performance_mode, auto_tune, workers_per_endpoint, poll_idle_sleep_ms, endpoint_blacklist_base_ms, endpoint_blacklist_max_ms, endpoint_outage_grace_ms, max_request_bytes_pre_encode, stream_connect_timeout_ms, stream_ping_interval_ms, stream_reconnect_backoff_ms, and the _comment_performance / _comment_advanced_performance blocks.
• server_config.example.json: auto_tune, performance_mode, active_drain_window_ms, long_poll_window_ms, upstream_dial_timeout_ms, coalesce_window_ms, coalesce_window_busy_ms, max_sessions, max_request_body_bytes, max_response_bytes_pre_encode.

tunnel_key placeholder — please also update the validator:
The placeholder swap to "REPLACE_WITH_64_HEX_CHARACTER_RANDOM_KEY" is fine in spirit (scripts/gen-key.sh doesn't actually exist in the repo — only deploy.sh, goose-relay.service, and goose-server.sh are under scripts/, so the existing hint sends users to a missing script), but it desyncs from the validator at internal/config/client.go:326, which still matches on the old string and still tells users to run scripts/gen-key.sh in its error messages (lines 327, 330, 334).

Could you include a small follow-up in this PR: update line 326 to match the new placeholder, and replace the three 'bash scripts/gen-key.sh' references in the error strings with 'openssl rand -hex 32' (which is what the README already instructs)? Keeps validator and example in sync and fixes the dangling-script reference at the same time.

Re: deadline removal in UrlFetchApp.fetch: was that intentional? Was a tradeoff explored? Apps Script's default deadline is shorter than the old 30s. If it was deliberate (e.g. so the call hangs up before Apps Script's 6-min execution cap rather than after a fixed 30s), worth a one-line comment in the .gs file.

Happy to merge as soon as the example-config diff is trimmed. Thanks again.

[poulcarlsen53]

Updated this PR based on your feedback:

• trimmed example config fields that are not read by internal/config yet
• kept the Apps Script relay changes
• restored the Apps Script fetch deadline
• synced the tunnel_key placeholder with the client config validator
• replaced the missing gen-key.sh guidance with openssl rand -hex 32

Verification:
go test -count=1 ./...
go vet ./...