Rename crate

[Flowneee]

Current name originally was made to refer fact that crate provide macro to generate OpenAPI operation definition using okapi types.

Why name is not good:

• Some people might think that this crate related to okapi project (which is false);
• Almost immediately was added spec builder, implementations and integrations for axum (and potentially for different frameworks);
• There is possibility that this crate will move from okapi to something different (or maybe fork okapi), because okapi is designed to support OpenAPI 3.0, while there is 3.1 version which use JSON Schema draft 2020-12 for describing schemas, and as I understand okapi not planning to support different versions;
• Name is ugly)

So I think it would be good to have something neutral, not referring to any specific project or crate, and and general enough to include different instruments for building HTTP API specifications from code.