RFR: 1910: Basic support for Bitbucket as a Forge

Fri May 12 20:15:31 UTC 2023

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.

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Commit messages:
 - SKARA-1910

Changes: https://git.openjdk.org/skara/pull/1521/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1521&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1910
  Stats: 533 lines in 8 files changed: 485 ins; 43 del; 5 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