feat(engineio-core): create engineioxide-core for common types

[Totodore]

Motivation

The socketioxide-core crate only use Sid and Str types from the engineioxide crate. Until now it wasn't a problem because engineioxide was a mandatory dependency for every dependent on socketioxide-core (socketioxide, adapter implementations). However as I'm currently thinking about making a socketioxide-redis-emitter (#459) it would be really nice to rely on socketioxide-core for some types (Packet, BroadcastOptions, Parser) but we should not pull all the rest of engineioxide when using this redis-emitter.

Solution

Isolate the Sid/Str types to an engineioxide-core crate and make socketioxide-core and engineioxide depend on it. This include a small breaking change as engineioxide now re-export Sid from the socket mod and not from the sid mod.

Side note

If one day we want to make an engineioxide-client and socketioxide-client this will also useful.