Allow overriding iTwin reality data URL for QA or Dev env

[ludovic-cheval-bentley-systems]

Description

Adds support for overriding the iTwin base URL when accessing Reality Data, aligning behavior with the existing override mechanism used for iTwin curated content.

This change improves flexibility for environments that require custom endpoints (e.g., DEV, QA, ...), ensuring consistent configuration capabilities across both curated content and Reality Data access.

For backward compatibility, when no overridden URL is given, behavior remains unchanged by defaulting to the existing Reality Data base URL for production.

Issue number or link

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• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

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• Review and run all parts of the test plan on this branch and verify it matches expectations.
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[azrogers]

Thanks @ludovic-cheval-bentley-systems!