feat(wasm): add initial token melting method

- add a new `melt_cashu_token` method to `mutiny-wasm`
- add a new `CashuMintUrlFailure` error variant too

wip(feat(node)): add new `melt_cashu_token` method

wip(feat(node)): implement new `melt_cashu_token` method

- add new `cashu_mint_client` to `MutinyWallet`
- add initial outline for calling `post_melt_tokens`

wip: melt_cashu_token method

wip: changed invoice generation for token amount

add doc strings and return invoices instead of bool

only add paid invoices

refetch invoice

melt cashu token changes