feat: Add configurable bitbucket builds api

[piotrkochan]

Adds configurable Bitbucket Builds API support to the existing notifyBitbucket Pipeline step for compatiblity with the Required Builds merge check.

The updated step:

• supports the repository-scoped Builds API available in Bitbucket 7.4 or newer
• adds a global Bitbucket build status API setting with Legacy as the default and Builds API (Bitbucket 7.4 or newer) as the alternative
• allows to override the global setting for each notifyBitbucket invocation with useBuildsApi: true / false
• requires bitbucketProjectKey and repositorySlug when the Builds API is enabled
• includes the build parent expected by Required Builds, using the multibranch project path for multibranch jobs and the job path for regular jobs
• keeps the existing notifyBitbucket step, its configuration options, and the legacy endpoint unchanged when the Builds API is not enabled

PR also adds a Jenkins Pipeline integration test suite and runs it from GitHub Actions

Fixes #429 #290

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