Add get_metadata() method and make query_metadata() raise KeyError consistently

Introduces a Pythonic get_metadata() method to BinaryView, Function, Project,
TypeArchive, and TypeLibrary classes. This method behaves like dict.get(),
returning a default value (None by default) when a key doesn't exist, instead
of raising a KeyError.

Additionally, updates query_metadata() in TypeArchive and TypeLibrary to raise
KeyError when a key is not found, making them consistent with BinaryView,
Function, and Project. Previously these two classes returned None on missing
keys. This breaking change is documented in the method docstrings.

This provides a more consistent and Pythonic API for querying metadata across
all metadata-supporting classes.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>