Add BitBucket Cloud scaffolding

[maciekbanas]

Description

Introduces 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).

New classes:

• EngineRestBitBucket — REST engine with BitBucket-style cursor pagination (next URL), repo listing, commit retrieval with client-side date filtering, contributors approximation from recent commits, and file tree browsing.
• GitHostBitBucket — REST-only host subclass. Overrides org/repo/commit retrieval to use REST, workspace validation, and repos data caching. Unsupported methods (get_issues, get_pull_requests, get_release_logs, get_files content, get_users) raise informative errors.

New exported function:

• set_bitbucket_host() — follows the same pattern as set_github_host() / set_gitlab_host(). BitBucket workspaces map to the orgs parameter.

Currently supports: get_repos(), get_commits(), get_orgs(), get_repos_trees().

Notable design decisions:

• No GraphQL engine — private$engines$graphql is NULL; all base class methods that delegate to GraphQL are overridden.
• Contributors approximated from unique authors in the last 100 commits (BitBucket has no /contributors endpoint).
• Commit additions/deletions set to NA (requires separate diffstat API call — deferred to get_commits() support #770).
• Token validation uses GET /user success (HTTP 200) since BitBucket doesn't return scope headers.

Note: roxygen2::roxygenise() needs to be run to regenerate man pages for set_bitbucket_host before merging.

Related Issue(s)

Closes #768
Part of #754

How to test

1. Unit tests: testthat::test_file(\"tests/testthat/test-set_host-BitBucket.R\")
2. Manual (requires a BitBucket Cloud app password with repository scope):

library(GitStats)
gs <- create_gitstats() |>
  set_bitbucket_host(
    token = Sys.getenv("BITBUCKET_PAT"),
    orgs = "your_workspace"
  )
gs |> get_repos()
gs |> get_commits(since = "2024-01-01", until = "2024-12-31")

3. Run R CMD check to verify no regressions.