RFR: 1910: Basic support for Bitbucket as a Forge [v3]

[Erik Joelsson]

> This patch adds very basic support for Bitbucket as a Forge, just enough to be able to use a Bitbucket repo as the source for mirroring.
> 
> In order to support ssh for bitbucket, I moved the `configureSshKey` method from `GitLabForgeFactory` to a new utility class, so that it could be shared more easily.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Review followup

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Changes:
  - all: https://git.openjdk.org/skara/pull/1521/files
  - new: https://git.openjdk.org/skara/pull/1521/files/0a81c6f6..c3977cc1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1521&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1521&range=01-02

  Stats: 34 lines in 3 files changed: 5 ins; 12 del; 17 mod
  Patch: https://git.openjdk.org/skara/pull/1521.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1521/head:pull/1521

PR: https://git.openjdk.org/skara/pull/1521