Consider making EndpointSpec public

[BRA1L0R]

Without this trait being public no user of this crate can implement their requests.

[ochrons]

In Cargo.toml there is an unused spec feature, which should be renamed to endpoint-spec to make it work.

[taladar]

With the cfg Warnings the compiler implemented recently there are some warnings about endpoint-spec uses in the code too so it is likely that spec was always intended to be endpoint-spec in the Cargo.toml