chore: nwaku should be a submodule of status-go

[Ivansete-status]

Description

nwaku should be a submodule of status-go.

The status-go's feature/nwaku-in-status branch expects to have the following structure, which is so far being created/copied manually:
.

Instead of doing that, we need to have a nwaku submodule, within status-go.

The libwaku.so should be compiled from within the status-go's Makefile. Notice that for libwaku.so to bring support to postgres, it should be compiled with make libwaku POSTGRES=1.

Once the new libwaku.so and libwaku.h location is defined, we will need to update the following paths to make it compile properly:
https://github.com/status-im/status-go/blob/94dc345d8ef5fb95fd855a665b1d32f3b0e32008/wakuv2/nwaku.go#L4-L7