feat(wallet_ffi): wallet_get_utxos, wallet_coin_join, wallet_coin_spl…

…it (#4244)

FFI join & split functionality + tests
rewrote coin_split function
fixed TariVector conversion bug
found a problem in the coin_join approach, need reconsideration
this commit is for review only before the final brushing/wrap-up
adding TariVector to substitute TariOutputs et al (WIP)
added coin join + initial unit test (still WIP)
added an output database shortcut for easier access from the wallet context, circumventing hops through async fall-through constructs; updated all usages - `cargo test` passing.
added FFI `wallet_get_utxos()`, `destroy_tari_outputs` + unit test
added a simple output querying function