Implemented rsa-sha2-512, rsa-sha2-256 host_key algs

[mfazekas]

Fixes: #712

[codecov-commenter]

Codecov Report

Merging #771 into master will decrease coverage by 0.03%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master     #771      +/-   ##
==========================================
- Coverage   95.80%   95.77%   -0.04%     
==========================================
  Files         155      155              
  Lines       10176    10192      +16     
==========================================
+ Hits         9749     9761      +12     
- Misses        427      431       +4     

Impacted Files	Coverage Δ
lib/net/ssh/transport/algorithms.rb	98.06% <ø> (ø)
lib/net/ssh/authentication/certificate.rb	94.05% <50.00%> (ø)
test/integration/common.rb	93.97% <50.00%> (-3.47%)	⬇️
lib/net/ssh/transport/openssl.rb	88.42% <80.00%> (-1.14%)	⬇️
lib/net/ssh/authentication/ed25519.rb	99.03% <100.00%> (ø)
lib/net/ssh/transport/kex/abstract.rb	96.72% <100.00%> (+0.29%)	⬆️
test/transport/kex/test_ecdh_sha2_nistp256.rb	100.00% <100.00%> (ø)
test/transport/test_algorithms.rb	99.56% <100.00%> (ø)
lib/net/ssh/buffered_io.rb	85.50% <0.00%> (+1.44%)	⬆️

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[fwininger]

@mfazekas thanks