RFR: Add Host::isMemberOf

[Erik Helin]

Hi all,

this patch adds a new method `Host::isMemberOf` that can be used to query a host (e.g. GitHub or Gitlab) if a specific user is a member of a particular group. I couldn't come up with a positive test for the method, since it is really hard to rely on a test user being a member of a particular group, but I could at least write a negative test.

## Testing
- [x] `sh gradlew test` passes on Linux x86_64

Thanks,
Erik

----------------

Commits:
 - 52633d26: Add Host::isMemberOf

Changes: https://git.openjdk.java.net/skara/pull/173/files
 Webrev: https://webrevs.openjdk.java.net/skara/173/webrev.00
  Stats: 70 lines in 5 files changed: 70 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/173.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/173/head:pull/173

PR: https://git.openjdk.java.net/skara/pull/173