Support more account operations (#150)

* Add missing fields in AccountInfo

* Add AccountService.GetAccountExternalIDs

Add the type AccountExternalIdInfo to serve as return value. The
naming (Id rather than ID) is picked to be consistent with the Gerrit
documentation.

* Add AccountService.QueryAccounts

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Co-authored-by: Andy Grunwald <andygrunwald@gmail.com>

Author: hanwen