[netstack] Check in API definition for network connectivity service.

Change-Id: I10e4b50eeb0587b6a45b8b4716ef4fef0a818d09
Tested:
- fx full-build this patch and https://fuchsia-review.googlesource.com/c/garnet/+/163768
- fx run-test netstack_connectivity_test

[fidl][net] FIDL interface for socket creation

A legacy interface to replicate the remote IO protocols.

NET-863

Test: None since nothing uses or implements this library yet
Change-Id: Iaa6e8782bf36876ae10a71a491404fd60ae11446

[fidl] Rename FIDL directories

These names more closely match what we publish in the SDK. Ideally, we'd
use . as the separator, but Zircon's build system cannot handle . in
directory names, so we use - instead.

Test: Builds.
Change-Id: Ia420a364d2accac39faebade8f3f8ed72088556a