Allow removing litep2p from dependencies

[nazar-pc]

litep2p causes multiple issues downstream for us at Subspace. Older release brought vulnerable ed25519-dalek version that we had to suppress in cargo audit, now I noticed it pulls a large number of extra dependencies, including OpenSSL that I was trying to avoid all this time and was pretty successful at it.

The fact that it it enabled by default and not possible to disablable is very undesirable, please consider making it an option just like it is possible to disable RocksDB right now for databases.