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
• [x] Feature
• [ ] Code style update
• [x] Refactor
• [ ] Other, please describe:

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

• [ ] Yes
• [x] 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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