Technical spec for wallet-dApp communication.

…ementation of it in Yoroi.

Type definitions are specified using flow types now instead of some
pseudo JS schema.

API errors are added to all API calls to handle things that can go wrong
for any call,e.g. internal error in the wallet, incorrect params
specified to API calls.

get_change_address() was added as an explicit API since it should be
easy to support and allows for an easier way during tx creation rather
than arbitrarily choosing an address from the used or unused API calls,
which aren't garuanteed to be non-empty.

add_external_box was removed as we are still unsure how any wallets
would be able to implement it in any meaningful way. This could be
reversed and have most wallets treat it as a no-op.

The id field has been removed, as well as the tx id/box id from outputs
which are now BoxCandidates.

This was only used before to help with interop with exisitng tooling but
now that sigma-rust no longer requires these there's no reason to keep
them around. They only made it more difficult to construct an unsigned
transaction. Now one can be a lot more easily constructed even without
using other tooling to calculate the IDs/etc.

typo in type name

Co-authored-by: Denys Zadorozhnyi <denys@zadorozhnyi.com>