[Tooling] Add Buildkite release pipelines and fastlane lanes for ReleasesV2

[iangmaia]

Fixes AINFRA-1806, AINFRA-1943

ReleasesV2 PR: https://github.a8c.com/Automattic/wpcom/pull/203624

⚠️ Note: this is still a draft as I need to test and refine the project with full integration.

Proposed changes

Adds CI automation for Studio releases, to be triggered from ReleasesV2. This is the companion PR to the wpcom ReleasesV2 configuration PR.

Fastlane lanes added

• code_freeze — Creates release branch from trunk, extracts translatable strings, delegates to new_beta_release
• new_beta_release — Bumps beta version, commits, pushes, creates GitHub prerelease, triggers release build
• finalize_release — Bumps to final version, creates draft GitHub release, triggers release build
• publish_release — Publishes draft GitHub release, creates backmerge PR to trunk
• new_hotfix_release — Creates hotfix branch from latest release tag, bumps version
• distribute_release_build — Distributes build artifacts (called from release-build-and-distribute pipeline)

Buildkite pipelines added

• release-pipelines/code-freeze.yml
• release-pipelines/new-beta-release.yml
• release-pipelines/finalize-release.yml
• release-pipelines/publish-release.yml
• release-pipelines/download-translations.yml
• release-pipelines/new-hotfix-release.yml
• release-build-and-distribute.yml — Build jobs extracted from the main pipeline.yml and triggered by Fastlane lanes to build Mac/Windows artifacts and distribute.

How it works

ReleasesV2 triggers a Buildkite build with a PIPELINE env var pointing to the release pipeline YAML added in this PR. The release pipeline runs the corresponding Fastlane lane.

For lanes that produce builds (code_freeze, new_beta_release, finalize_release), we use buildkite_add_trigger_step to trigger a separate release-build-and-distribute build that handles Mac/Windows builds and artifact distribution.

Testing instructions

• Test lanes locally (e.g. code_freeze) locally (without skip_confirm) to verify prompts and flow
• Trigger a test build from ReleasesV2 on a sandbox to verify end-to-end integration; if needed, cancel Buildkite jobs once you validate them