Introduce the `astro api` top-level command with two subcommands:

  astro api airflow — make requests to the Airflow REST API (local or
  deployed), with automatic version detection (2.x, 3.0.x, 3.0.3+) and
  OpenAPI spec resolution.

  astro api cloud — make requests to the Astro Cloud platform API using
  the current context's bearer token.

Both subcommands support:
  - Endpoint discovery via `ls` and `describe` (parsed from OpenAPI specs)
  - Calling endpoints by operation ID or raw path
  - Pagination (per-page streaming or --slurp into a single array)
  - Response caching with TTL and automatic stale-entry cleanup
  - jq filters and Go-template output formatting
  - Colored JSON output
  - --curl flag to emit equivalent curl commands
  - Magic field syntax for request bodies (@file, :=json, =string)
  - Custom headers and path-parameter overrides

Supporting packages:
  - pkg/openapi: OpenAPI spec fetching, caching, endpoint indexing,
    Airflow version-range mapping, and schema introspection.

Includes unit tests for request handling, output formatting, field
parsing, OpenAPI version mapping, endpoint indexing, and caching.