chore: gate embed p2p transport on cargo feature

[kaichaosun]

Refactor on feature usage in embed p2p transport, use a consistent cargo feature, not env-dependent cfg

[jazzz]

What is the plan for chat-cli? Does this change aim to remove the the transport choice within chat-cli and commit to using embedded λDelivery?

The existing approach, (supporting both cargo feature declaration as wells as envvars) was needed was to support the current setup of chat-cli, which allowed the delivery mechanism to be chosen at compile time via envvars.

[kaichaosun]

Another PR after this change:
https://github.com/logos-messaging/libchat/compare/refactor-embed-transport-feature...more-transport-features?expand=1

I don't see any hard blocker to keep the env feature.

[jazzz]

I don't see any hard blocker to keep the env feature.

If you mean that you want to remove the "env feature" from the cli app then I don't see any issue with this change.

I'd further suggest these changes for the future PR.

• removing the '--transport' flag all together. Embedded delivery needs to be compiled anyway. Remove the added complexity and simplify usage.
• Logos-delivery should be reserved for logos-delivery via logos-core. Use embedded* to refer to linking directly to the compiled library.