Handle BitBucket API

[maciekbanas]

Add BitBucket Cloud (api.bitbucket.org/2.0) as a third supported Git host alongside GitHub and GitLab.

BitBucket Cloud uses REST API v2 only (no GraphQL). Key differences from GitHub/GitLab:

• Workspaces instead of organizations/groups
• Cursor-based pagination via next URL field
• No native releases concept (tags only)
• Issue tracker is optional per-repo
• No additions/deletions in commit responses (requires separate diffstat call)
• Single language field per repo (no language breakdown)

Sub-issues

• Core scaffolding — EngineRestBitBucket, GitHostBitBucket, set_bitbucket_host() #768 — Core scaffolding: EngineRestBitBucket, GitHostBitBucket, set_bitbucket_host()
• get_repos() and get_orgs() support #769 — get_repos() and get_orgs() support
• get_commits() support #770 — get_commits() support
• get_pull_requests() support #771 — get_pull_requests() support
• get_issues() support #772 — get_issues() support
• get_files() and file tree support #773 — get_files() and file tree support
• get_users() and get_release_logs() support #774 — get_users() and get_release_logs() support