Add support for additional user attributes to DataSourceRealm

[cklein05]

@michael-o Here's the patch to add support for additional user attributes to DataSourceRealm.

[rmaucher]

Since I am looking at open PRs once in a while, I merged the pluming code for the user attributes with fd5b0fb6ea4aeeea8a3b3c95064b803068d531f9 I added more of the code to RealmBase, since I couldn't figure out why realms would need to do something different (and if they somehow do overriding works ...).

Status update: Still not motivated by having to maintain the JDBC code though ...