feat: Add support for remote signing keys

[awly]

When used as a library, nfpm.PackageSignature.SignFn can be set as an alternative to KeyFile. This allows arbitrary signing key implementations, like a remote signing server.

Updates https://github.com/tailscale/tailscale/issues/1882

[awly]

@caarlos0 thanks! I think you'll need to approve CI checks to run and merge the PR, since I don't have permission to do either ;)

[codecov[bot]]

Codecov Report

Merging #695 (823a95e) into main (577ae45) will increase coverage by 0.21%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #695      +/-   ##
==========================================
+ Coverage   74.80%   75.02%   +0.21%     
==========================================
  Files          10       10              
  Lines        2385     2406      +21     
==========================================
+ Hits         1784     1805      +21     
  Misses        425      425              
  Partials      176      176              

Files Changed	Coverage Δ
nfpm.go	85.00% <ø> (ø)
apk/apk.go	73.79% <100.00%> (+0.55%)	:arrow_up:
deb/deb.go	72.42% <100.00%> (+0.61%)	:arrow_up:
rpm/rpm.go	70.92% <100.00%> (+0.41%)	:arrow_up:

[caarlos0]

merged, thank you @awly (I love tailscale btw 🙏)