Passes references to the public and secret keys to sign/verify

[sr-gi]

sign/verify should not take ownership of the keys. Didn't pay enough attention when originally implementing this.

[codecov]

Codecov Report

Merging #974 (c1d2d15) into main (ecddfe1) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #974      +/-   ##
==========================================
- Coverage   90.65%   90.64%   -0.01%     
==========================================
  Files          60       60              
  Lines       30398    30398              
==========================================
- Hits        27558    27555       -3     
- Misses       2840     2843       +3     

Impacted Files	Coverage Δ
lightning/src/util/message_signing.rs	93.10% <100.00%> (ø)
lightning/src/ln/channelmanager.rs	83.85% <0.00%> (-0.05%)	⬇️
lightning/src/ln/functional_tests.rs	97.12% <0.00%> (-0.04%)	⬇️

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