cleanup: shrink usage of GaxiosOptions as argument to HTTP2.Request

[ddelgrosso1]

While investigating googleapis/google-api-nodejs-client#3187 I noticed that the http2.request function takes GaxiosOptions as the configuration. However, a lot of these GaxiosOptions do not have a 1-to-1 mapping in ClientHttp2Session. We might want to rethink using GaxiosOptions as the argument type and instead create options that align better with functionality available in ClientHttp2Session.