Meta::thriftIDL endpoint

[abhinav]

We already have the health endpoint but we're missing the thriftIDL endpoint.

struct ThriftIDLs {
    // map: filename -> contents
    1: required map<filename, string> idls
    // the entry IDL that imports others
    2: required filename entryPoint
}

service Meta {
    HealthStatus health()
    ThriftIDLs thriftIDL()
}