C FFI to change the gateway URL on a per-sphere context basis (without re-initializing Noosphere)

[bfollington]

After landing https://github.com/subconsciousnetwork/subconscious/pull/755 to fix https://github.com/subconsciousnetwork/subconscious/issues/693 we realised that we can avoid having to deinit and reinit Noosphere at all if we introduce a new FFI method to update the gateway URL and rebuild the relevant internal Noosphere state.

[cdata]

Here is how we support this in the underlying Rust API:

https://github.com/subconsciousnetwork/noosphere/blob/8b806c5462b5601a5f8417a6a20769b76b57ee6a/rust/noosphere-sphere/src/context.rs#L155-L168