[secp256k1] refactor nativeECDSABuffer to a more generic name
Summary: refactor to a generic name since know we use schnorr sigs
Test Plan: java tests
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox
Subscribers: schancel
Differential Revision: https://reviews.bitcoinabc.org/D2756