fix: authentication for analytics

[arcoraven]

PR-Codex overview

This PR updates the ANALYTICS_DEFAULT_HEADERS object in src/shared/utils/usage.ts to use the env variable for ENGINE_VERSION and adds a new header for THIRDWEB_API_SECRET_KEY.

Detailed summary

• Changed the value of x-sdk-version from process.env.ENGINE_VERSION to env.ENGINE_VERSION.
• Added a new header x-secret-key with the value env.THIRDWEB_API_SECRET_KEY.

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