feat(cli): openapi support assigning servers to an environment

[armandobelardo]

Exposes an x-fern-server to add both name and env, and any additional config we want in the future

 servers:
   - url: https://api.example.com
     x-fern-server:
        name: api
        environment: Production

[dsinghvi]

@armandobelardo going to close this out since we now support this in generators.yml