[secp256k1] add schnorr sign jni binding

Summary: add the jni binding for schnorr sign

Test Plan: run the single test but ill add more tests. any suggestions where i can take failing and positive tests is welcome

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, jasonbcox

Subscribers: jasonbcox, Fabien, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2755