DRIVERS-2836 OIDC: Clarify that TOKEN_RESOURCE must be url-encoded

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Please complete the following before merging:

• [x] Update changelog.
• [ ] Make sure there are generated JSON files from the YAML test files.
• [x] Test changes in at least one language driver.
• [ ] Test these changes against all server versions and topologies (including standalone, replica set, sharded clusters, and serverless).

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Tested with https://github.com/mongodb/mongo-python-driver/pull/1616

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@sanych-sun @katcharov I've updated the description and tests based on feedback

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Updated tests in https://github.com/mongodb/mongo-python-driver/pull/1620