Split crate

[Flowneee]

Currently project consist of 2 crates - main (okapi-operation) and proc-macro (okapi-operation-macro). I think it is good idea to split it into multiple crates

• -core: core types and traits (like ToResponses, ToMediaTypes) + impls for framework-specific types (like axum::extract::Json) under feature flags (because of orphanage rules)
• -<framework>: additional framework-specific things, for example for axum it could be Router replacement;
• -macro: proc-macro crate as is + framework-specific features under feature flags
• top-level crate: gather and expose all other crates with framework-specific feature under feature flags