Add full support for service account impersonation

[quartzmo]

Regarding the support for service account impersonation that was discussed in #346 and that I added in #347, I noticed that googleapis/google-auth-library-java/pull/613 and golang/oauth2/pull/468 go beyond accepting a token obtained externally from projects.serviceAccounts.generateAccessToken and actually perform the RPC to obtain the token when provided with the appropriate service account credentials. Should this library do the same?

[konalegi]

@quartzmo do you plan to implement this feature?

[konalegi]

@quartzmo I've tried to implement service account impersonation with ADC, It worked locally, although I have no idea if I'm moving right direction, since not very familiar with all this kitchen. If you have a time could you please have a look and give suggestion where to move next.
https://github.com/googleapis/google-auth-library-ruby/pull/485/files
Thank you!