fix: avoid global mutation in BigQueryOptions.client_endpoints_override

[tswast]

This fixes a common "gotcha" in Python. Before this fix, mutations to this dictionary will affect all instances of the BigQueryOptions class, not just the current instance.

See: https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

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Fixes internal issue 389087778 🦕

[GarrettWu]

Thanks for the fix!