This does the following:
* Adds a rfc6979 argument to test_framework/key.py's sign_ecdsa to
select (deterministic) RFC6979-based nonce generation.
* Add a flag in feature_taproot.py's framework called "deterministic".
* Make the Schnorr signing in feature_taproot.py randomized by default,
reverting to the old deterministic (aux_rnd=0x0000...00) behavior
if the deterministic context flag is set.
* Make the ECDSA signing in feature_taproot.py use RFC6979-based nonces
when the deterministic context flag is set (keeping the old randomized
behavior otherwise).
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