L7 rebased

Adds coverage of the TargetHttpProxyApi, UrlMapApi, GlobalForwardingRuleApi, and BackendServiceApi.

Also adds a complete License file to the top level.

I apologize for the large size of this PR.
For input objects, we can add builders in the mean time. Ex
think AttachDisk, except adding AttachDisk.Builder with straight-forward
builder methods for each field.