Fix UserInfo Gson deserialization and add ProGuard rules

[Subterrane]

Summary

• Add @SerializedName annotations to UserInfo.kt so Gson correctly maps snake_case JSON keys (preferred_username, given_name, family_name, updated_at) to camelCase Kotlin properties
• Change updatedAt type from String? to Long? to match the OIDC spec (Unix epoch timestamp)
• Add ProGuard/R8 consumer rules to prevent field stripping when consumers enable minification
• Add unit tests for UserInfo deserialization (snake_case mapping and null handling)
• Update Java demo app to handle the Long type change for updatedAt

Context

This supersedes PRs #6, #13, and #16, which all independently identified and attempted to fix the same bug: OIDC UserInfo fields with snake_case names were always null after deserialization. This PR combines the correct parts of each:

PR	What it got right	What it missed
#6 (Alqueraf)	@SerializedName annotations	updatedAt typed as Int? (overflows in 2038)
#13 (jkmaina)	@SerializedName annotations	Left updatedAt as String?; bundled unrelated JSONConvertable interface
#16 (lub0s)	@SerializedName + Long? type + demo updates	SimpleDateFormat thread-safety issue in demo

Test plan

• Verify UserInfoDeserializationTest passes (snake_case field mapping + null handling)
• Verify existing NetworkClientTest still passes
• Verify Java demo app compiles with the updatedAt type change