Add ImpersonatedCredentials

[salrashid123]

Addresses #210

Allow for ImpersonatedCredentials in core auth library set. Instructions to repro or to run system tests:

# Create source identity:
# gcloud iam service-accounts create source-serviceaccount --display-name="Source Identity"
# gcloud iam service-accounts keys  create svc-src.json --iam-account=source-serviceaccount@mineral-minutia-820.iam.gserviceaccount.com


# Create target identity
# gcloud iam service-accounts create target-serviceaccount --display-name="Target Identity"

# Allow source to impersonate target
# gcloud iam service-accounts add-iam-policy-binding target-serviceaccount@mineral-minutia-820.iam.gserviceaccount.com --member='serviceAccount:source-serviceaccount@mineral-minutia-820.iam.gserviceaccount.com' --role='roles/iam.serviceAccountTokenCreator'

# Add resource ACL to target
# gcloud projects add-iam-policy-binding mineral-minutia-820 --member='serviceAccount:target-serviceaccount@mineral-minutia-820.iam.gserviceaccount.com' --role='roles/storage.admin'

# Test the app below

# now create a GCE instance of the same source identity.
# comment out the service account section and uncomment the ComputeCredential section (as ADC)
# gcloud compute instances create impersonate-test --service-account=source-serviceaccount@mineral-minutia-820.iam.gserviceaccount.com --scopes=https://www.googleapis.com/auth/iam
# Upload the source below to GCE and rerun the test

output of unittests ran:

[INFO] Tests run: 9, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.066 s - in com.google.auth.oauth2.ImpersonatedCredentialsTest
``

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1b5f8ac). Click here to learn what that means.
The diff coverage is 73.26%.

@@            Coverage Diff            @@
##             master     #211   +/-   ##
=========================================
  Coverage          ?   76.03%           
  Complexity        ?      327           
=========================================
  Files             ?       21           
  Lines             ?     1456           
  Branches          ?      158           
=========================================
  Hits              ?     1107           
  Misses            ?      270           
  Partials          ?       79

Impacted Files	Coverage Δ	Complexity Δ
...om/google/auth/oauth2/ImpersonatedCredentials.java	73.26% <73.26%> (ø)	11 <11> (?)

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[chingor13]

Thanks for this!

[chingor13]

Thank you! I can handle any other cleanup separately.