Use `FrozenDocumentLoader` when HTTP loaders are unavailable

[anatoly-scherbakov]

At import time, PyLD currently selects RequestsDocumentLoader when requests is installed, AioHttpDocumentLoader when aiohttp is installed, and otherwise a dummy loader that raises on use.

Consider using FrozenDocumentLoader as the third fallback so bundled contexts remain available when neither HTTP dependency is installed.

This would change the fallback behavior for URLs outside BUNDLED_CONTEXTS, so assess compatibility, error behavior, default-loader documentation, and tests before implementation.

Origin: #301 (comment)