for example to store a 'return to' url to redirect the user to after they login

DO NOT SEND ANY SECURITY FIX HERE. Please read "Security Reporting" section
on README.

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 - assuming no-one us using a key named 'extra_state' already for integrations with their IDP's

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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