[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