Extract load_key construction to separate method

[borislaviv]

This approach allows implementors to define custom key selection strategy without need to override the entire parse_id_token method.

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What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

(If no, please delete the above question and this text message.)

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