RFR: Make it possible to cancel a Check

[Erik Helin]

Hi all,

this patch became a bit larger than I initially thought. I added a new `CheckState` - `CANCELLED` and a new method to `CheckBuider` - `cancel()`. However to fully support this I needed to quite a bit of refactoring in both `GitHubPullRequest` and `GitLabMergeRequest` so that `createCheck` allows checks with all kinds of states to be created.

## Testing
- [x] `sh gradlew test` passes on Linux x86_64
- [x] Added one new unit test
- [x] `sh gradlew test` with credentials mostly passes against both GitHub and GitLab (there are a few known failures)

Thanks,
Erik

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Commits:
 - ed66f45c: Make it possible cancel a Check

Changes: https://git.openjdk.java.net/skara/pull/180/files
 Webrev: https://webrevs.openjdk.java.net/skara/180/webrev.00
  Stats: 245 lines in 5 files changed: 149 ins; 48 del; 48 mod
  Patch: https://git.openjdk.java.net/skara/pull/180.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/180/head:pull/180

PR: https://git.openjdk.java.net/skara/pull/180