Expose all supported signature algorithms list

[stepancheg]

Follow-up to #187.

I agree to license my contributions to each file under the terms
given at the top of each file I changed.

[codecov]

Codecov Report

Merging #192 (ad9057d) into main (07306a6) will increase coverage by 0.10%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
+ Coverage   71.32%   71.42%   +0.10%     
==========================================
  Files          15       15              
  Lines        1395     1400       +5     
==========================================
+ Hits          995     1000       +5     
  Misses        400      400              

Impacted Files	Coverage Δ
src/lib.rs	15.09% <0.00%> (ø)
src/signed_data.rs	100.00% <ø> (ø)
tests/integration.rs	100.00% <100.00%> (ø)

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[briansmith]

Sorry, I know I asked you specifically to make this change, but now I remember why we don't expose this: In the past, and probably in the future, we've implemented algorithms (e.g. SHA-1-based algorithms) that people probably shouldn't use, and we don't want to encourage people to accidentally enable them. So I'm just going to close this and I'll comment in your other PR.

[stepancheg]

Maybe could be called something like "all modern algorithms", not "all algorithms", but fine.