RFR: forge: add builder for HostUser

[Erik Helin]

Hi all,

please review this patch that adds "builder" for the class `HostUser`. The builder also generalizes the concept of
fetching missing information for a `HostUser` so that any username, id and full name now can fetched lazily.

Thanks,
Erik

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Commit messages:
 - forge: add builder for HostUser

Changes: https://git.openjdk.java.net/skara/pull/890/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=890&range=00
  Stats: 125 lines in 5 files changed: 63 ins; 15 del; 47 mod
  Patch: https://git.openjdk.java.net/skara/pull/890.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/890/head:pull/890

PR: https://git.openjdk.java.net/skara/pull/890