Introducing on-demand Pipelines: run pipelines via API

Introduces 'on-demand pipelines' which allow programmatic generation and execution of pipeline definitions via the Bitbucket Pipelines API. This enables dynamic pipeline creation based on context (e.g., services, environments, tests) without committing YAML to a repository. Supports triggering via API requests with YAML in the body, targeting branches or commits, and passing variables. Security is enforced by requiring write access to the target branch. Use cases include ad-hoc quality gates, safe experimentation, event-driven automation, and dynamic workflows.