[runtime] Enhancement: Adding transport trait

[iyzhang]

This PR adds a transport trait for high level network transport. In contrast to our network runtime, which abstracts the lowest level network API (i.e., only raw ethernet send and receive), the NetworkTransport trait abstracts a high-level network API including sockets. There are 3 commits that do the following:

1. Add a transport generic to the CatnapQueue
2. Add a transport generic to the CatnapLibOS
3. Move the CatnapLibOS and CatnapQueue to a common NetworkLibOS and NetworkQueue in the demikernel/libos/network directory and remove all CatnapLibOS and CatnapQueue code.

[anandbonde]

Looks good.