Chore: update polkadot/api to 10.9.1

[bvotteler]

Resolves #670

DRAFT

Changes made:

• moved type generation into separate module, using esnext to work around Typegen Usage which worked with 9.14.2 doesn't work with 10.1.2 polkadot-js/api#5540
• polkadot/api is now hoisted transitively for interbtc-api from the submodule (api-augment)
• fix broken imports, and type name change
• hacky-but-works™ import of json definitions from @interlay/interbtc-types

TODO:

• Tests pass
• Review and clean up leftover comments, temporary code pieces, remove unused files

[bvotteler]

Sadly, with this approach I could not make the mocha tests work in their current form, either.
Closing this draft and continuing to migrate mocha/chai/sinon to jest in PR #672