RFR: forge: add methods addCollaborator and canPush

[Erik Helin]

Hi all,

please review this patch that adds two methods to `HostedRepository`:

- `addCollabarator`
- `canPush`

These methods can be used to check and grant access for a user to a particular hosted repository.

Thanks,
Erik

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Depends on: https://git.openjdk.java.net/skara/pull/1046

Commit messages:
 - forge: add methods addCollaborator and canPush

Changes: https://git.openjdk.java.net/skara/pull/1047/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1047&range=00
  Stats: 62 lines in 5 files changed: 62 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1047.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1047/head:pull/1047

PR: https://git.openjdk.java.net/skara/pull/1047