* Add high-level helpers for using Musig2 with Taproot

When using Musig2 for a taproot key path, we can provide simpler helper
functions to collaboratively build a shared signature for the spending
transaction. Those helper functions hide the low-level details of using
an opaque key aggregation cache or signing session. This comes with a
small performance penalty, as we recompute the key aggregation cache.

We also document the exposed APIs, import more tests from the official
test vectors, and make APIs safe: they should never throw exceptions,
except when invalid public keys are provided as inputs (which should be
verified by the caller beforehand).

* Move taproot signing helpers to `Transaction.kt`

This is more consistent with the existing `signInput` helpers.