Add per-project service account auth

[jfreden]

• ServiceAccountToken constructor was changed to public, because the token had to be in core due to it being part of the interface in security extension, that's part of core. A lot of tests already rely on the constructor being package private. An aleternatie would be to move the ServiceAccountService to core, but that involves moving all service account definitions to core, which feels like too big of a refactor to justify. Ideally the tests should not use an interface that's not public and instead use a mock.
• Add tests
• Should we validate that no cluster level service account file is configured?
• Should we validate that no index level service account is configured?
• Token source, should we add some tests for it?
• Revert change to some of the token things