Passkeys: `JSON.stringify()` fallback for various password managers

[guardrex]

Per dotnet/aspnetcore#63078 (comment).

https://github.com/dotnet/aspnetcore/pull/62917/files

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Document Details

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• ID: f44a13ca-c7a0-490a-9251-931ff16554f2
• Version Independent ID: f44a13ca-c7a0-490a-9251-931ff16554f2
• Platform ID: c5da9dfd-32c6-e966-5a91-9426d957e69f
• Content: Enable Web Authentication API (WebAuthn) passkeys
• Content Source: aspnetcore/security/authentication/passkeys/index.md
• Service: aspnet-core
• Sub-service: security
• GitHub Login: @guardrex
• Microsoft Alias: wpickett