Update IamClient to allow getting service account keys by type.

[ahoying]

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[codecov]

Codecov Report

Merging #657 into dev will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #657      +/-   ##
==========================================
+ Coverage   83.62%   83.64%   +0.01%     
==========================================
  Files         165      165              
  Lines        8313     8321       +8     
==========================================
+ Hits         6952     6960       +8     
  Misses       1361     1361

Impacted Files	Coverage Δ
google/cloud/security/common/gcp_api/iam.py	100% <100%> (ø)	⬆️

[blueandgold]

Nice!